There has been a large amount of commentary on John 18:6, and there is no settled explanation of why the soldiers and police (if indeed it is meant to be inclusive of all sent to arrest Him) stepped back and fell to the ground.
Perhaps the best commentary I have found on this passage is Meyer's New Testament commentary. He writes among other things
"[T]he falling to the ground of itself, and the circumstance that the
text designates those who fell down generally and without an
exception, so that even the Roman soldiers are to be understood along
with the rest, justifies the view of the ancient commentators, also
adopted by Strauss (who, however, as also Scholten, views the matter
as unhistorical), Ebrard, Maier, Luthardt, Hengstenberg, Godet, that
it was a miraculous result of the power of Christ (Nonnus:
οἰστρηθέντες ἀτευχέϊ λαίλαπι φωνῆς). Christ wished, before His
surrender, to make known His might over His foes, and thus to show the
voluntariness of His surrender."
However, there are those who hold it was not miraculous. Ellicott is an example of this, of which he writes among other things
"There is nothing in the narrative to suggest that our Lord put forth
miraculous power to cause this terror. The impression is rather that
it was produced by the majesty of His person, and by the answer which
to Jewish ears conveyed the unutterable name, “Jehovah” (I AM)."
Cambridge doesn't take sides, saying
"Whether this was the natural effect of guilt meeting with absolute
innocence, or a supernatural effect wrought by Christ’s will, is a
question which we have not the means of determining."
Barnes gives some speculative psychological details to fill out the scene from a non-miraculous perspective, and says
"The cause of their retiring in this manner is not mentioned. Various
things might have produced it. The frank, open, and fearless manner in
which Jesus addressed them may have convinced them of his innocence,
and deterred them from prosecuting their wicked attempt. His
disclosure of himself was sudden and unexpected; and while they
perhaps anticipated that he would make an effort to escape, they were
amazed at his open and bold profession. Their consciences reproved
them for their crimes, and probably the firm, decided, and yet mild
manner in which Jesus addressed them, the expression of his unequalled
power in knowing how to find the way to the consciences of men, made
them feel that they were in the presence of more than mortal man.
There is no proof that there was here any miraculous power, any mere
physical force, and to suppose that there was greatly detracts from
the moral sublimity of the scene."
Gill's holds a combination of these factors, and says
"Immediately upon his speaking these words, which were delivered with
so much majesty and authority, and were attended with such a divine
power: they went backward, and fell to the ground"
Interestingly, Expositor's says
"This might have been considered a fulfilment of Psalm 27:2"
which is
"When the wicked came upon me to devour my flesh, my enemies and foes
stumbled and fell."
and would be seen as another incident supporting Jesus being the Messiah.
Conclusion
If you believe with Meyer that the 'they' refers to everyone falling to the ground, including not just the leaders of the contingent but all the police and all the Roman troops, it seems difficult to believe something miraculous was not involved.
That Jesus surprised the contingent, or spoke majestically, or that certain listeners might have heard an invocation of God's name ('I am'), does not seem sufficient to explain the response. Rather, this interpretation would suggest a wave of power going out, similar indeed to how Jesus calmed the waters in the boat, demonstrating his authority over nature. Here, He is demonstrating his authority over those who would arrest him, and so showing
He is doing so voluntarily.
Another possibility along similar lines is that, unlike the control of nature, the response is due to an unveiling of divine glory, perhaps like the response to angels (cf. Matthew 28). On this view, the 'I am' coincides with an unveiling of Jesus' majesty, and causes the entire contingent to fall back and over.
If, however, you hold the 'they' refers to just some of the people, then a more natural explanation seems more plausible. Perhaps some were surprised, or taken aback at Jesus' use of 'I am' (personally, I find this theory weak, as a beggar uses the exact same term at John 9, and no one seems taken aback - although obviously the context is different here), or their consciences convicted them.
My best guess, agreeing with Meyer, is that the large number of leaders, police, and Roman troops were knocked over supernaturally, coinciding with Jesus' use of the term 'I am he', in order to demonstrate that Jesus was doing so voluntarily.
