Judges is full of parables like this, with many layers of meaning.
Interpreting the parable within the story
The key to unlocking this episode lies in the curse itself, which describes the history as a parable, so we should understand the parable to get the deeper meaning of the history:
Judges 9:8–15 (KJV 1900)
The trees went forth on a time to anoint a king over them; and they
said unto the olive tree, Reign thou over us.
But the olive tree
said unto them, Should I leave my fatness, wherewith by me they honour
God and man, and go to be promoted over the trees?
And the trees
said to the fig tree, Come thou, and reign over us.
But the fig
tree said unto them, Should I forsake my sweetness, and my good fruit,
and go to be promoted over the trees?
Then said the trees unto the
vine, Come thou, and reign over us.
And the vine said unto them,
Should I leave my wine, which cheereth God and man, and go to be
promoted over the trees?
Then said all the trees unto the bramble,
Come thou, and reign over us.
And the bramble said unto the trees,
If in truth ye anoint me king over you, then come and put your trust
in my shadow: and if not, let fire come out of the bramble, and devour
the cedars of Lebanon.
When analyzing a parable, we should follow the Hermeneutics of Jesus and first identify the players.
The fruit-bearing trees are the righteous ones. They lack nothing. They are perfectly content being exactly who they are. They are planted. They want for nothing and have no desire other than to enjoy being who they already are, and having what they already have.
The trees, in the history, are the people of Shechem. But the deeper meaning is they are just people. It's man. In stark contrast to the fruit-bearing trees, they are restless. They see a lack. And whenever you see a lack, you make something outside of you rule over you, because it decides whether you are happy or not, whether you are successful or not.
The trees try to turn to the righteous and be told what to do, in order to satisfy their lack. But the righteous have no desire to tell them what to do, because the message of the righteous is always "be planted and bear fruit".
So the trees turn to the bramble. A bramble is just a thorn-bush, and elsewhere I've argued that the thorns represent vain thoughts. Thus the thorn-bush is the carnal mind. The restless trees, not obtaining any satisfaction from the righteous, turn to their own vain thoughts for comfort. They make up some plan.
Then the thornbush invites them to "put your trust in my shadow" - which is absurd, because thorns have a terrible shadow. That is, putting your trust in the plans produced by your own mind is like counting on the shadow of a thornbush. It is foolish.
And then comes the threat "and if not, let fire come out of the bramble, and devour the cedars of Lebanon", which is again equally absurd, that a tiny thornbush would devour the cedar of Lebanon. That such a small and impotent thing as the carnal mind would destroy a mighty creation of God.
And yet this is exactly what happens!
So the parable is emphasizing how terribly foolish the whole episode is, that someone of the station of a tree should ask a thornbush to rule over him. Likewise that the men of Shechem would ask an interloper to rule over them. But that type of foolishness is what happens.
Interpreting the story
The story adds some other elements, the most important of which is the stone. Abimelech kills 70 brothers on one stone. What a strange thing to do. And then he himself is killed when a stone thrown by a woman falls from the sky and crushes his head -- an allusion to the seed of the serpent's head being crushed by the seed of the woman.
So we have an association of the thornbush, which is the carnal mind, with the seed of the serpent, which is a common allusion in scripture.
Why did Abimelech kill 70 on one stone? The stone, in the bible, represents Christ. Killing 70 on one stone is suggestive of a type of makeshift altar. E.g. Abimelech is trying to defile the stone, e.g. defile Christ, as murdering someone and pouring their blood on the altar is the worst possible sort of defilement (worse even that smearing pig's blood on an altar, if you can believe it). Thus Abimelech is not attacking the 70, he is attacking the stone, and the 70 are collateral damage. They are tools to try to attack the stone. The carnal mind always tries to slander and attack Christ in you. But it is powerless to damage the stone, and the stone ends up crushing Abimelech's head.
Another point. The people quickly tire of Abimelech and then seek someone else to be their king. E.g. the thornbush doesn't really provide very good shade. Vain thoughts cannot satisfy the inner restlessness, they serve only to gin up chaos so the divine fire will destroy the vain thoughts together with the feeling that you have a lack. And if not, another thornbush will appear and another round of divine fire is needed.
The last point, in his dying breath, Abimelech asks to be run through with a sword so that no one will know a woman killed him. This is when your idea collapses, it wont work, but instead of acknowledging that God destroyed the idea, because it was a vain thought, the last gasp of the carnal mind is to try to say "well, it might have worked but this other thing happened." So that in the future, another Abimelech will come along - another idea -- and you get to go through it all again, never being at peace and just being planted, happy with who you are and seeing no lack.
There are many other aspects to this that can also be plumbed, but this should be enough to unlock the riddle.