Hosea 10.8
While a number of the Hebrew prophetic books look at neighboring nations, the entirety of the book of Hosea is concerned with one subject: the tumultuous relationship between God and Israel.
The book opens with God instructing Hosea to marry a prostitute, with her adultery being used as an illustration of Israel's faithlessness to God, especially as related to their worship of idols. Consequently, Hosea's children receive names that symbolize an imminent judgment upon the nation. Afterward, God tells Hosea the judgment will pass and the destruction on Israel will be reversed. This theme of the punishment and restoration of Israel continues throughout the book, with the important point that punishment comes at the hands of Assyria, the chief antagonist of Israel in that era.
When we get to chapter 10, the theme has been ingrained in the reader. Here are key points in the chapter that indicate we're still on the topic of punishment for idolatry:
[Israel's] heart is false;
now they must bear their guilt.
The LORD will break down their altars
and destroy their pillars.
Ephraim will be put to shame,
and Israel shall be ashamed of his idol.
The high places of Aven, the sin of Israel,
shall be destroyed.
Thorn and thistle shall grow up
on their altars,
and they shall say to the mountains, "Cover us,"
and to the hills, "Fall on us."
— (ESV translation)
So in this full context, the 'they' (of 'they shall say...') are 'Israel'. With Israel's altars destroyed and overgrown with wild plants, the people will call out to the mountains and hills to hide them, even to crush them, rather than face God's wrath through his instrument Assyria.
Luke 23.30
The wording attributed to Jesus in Luke 23.30 is not identical to the existing Septuagint translation of Hosea 10.8, but the sentence structure is the same and the key words are present. What Luke says (via Jesus) is simply too similar to Hosea 10.8 to be a coincidence.
There is a related question here on Luke 23.31. In my response there, I wrote of Luke 23.28-31:
The saying ... fits into the eschatology of the Synoptic Gospels, which is largely focused on the 70 AD destruction of Jerusalem (cf. Luke 21).
People in Jerusalem (represented by the women) were mourning Jesus' death, which they saw as an injustice. Jesus agrees, warning them to weep not for him, but for their future generation ('your children'). He tells them that the injustice was occurring against a living tree: himself. But he ominously warns what will happen to a dead tree, people who are genuinely deserving of the kind of punishment he was about to face. Within the eschatology of the Synoptics, Jesus is here implying a divine judgment will come within that generation, which he likens to a dry tree, which will go up in flames. This concept is further corroborated by the quotation of Hosea 10.8, a passage about the destruction of Israel, in Luke 23.30.
Luke 23.30 invokes Hosea 10.8 because he saw first-century Jerusalem as deserving punishment in a similar manner to that of the idolatrous ancient Israel. However, the context in Luke doesn't indicate the punishment was for idolatry. Rather, it is primarily (or solely) because Jerusalem played a part in the crucifixion of Jesus.
