(Probably) Yes, but it will trigger only for the first attack.
As you mentioned, it will trigger for the first hit.
Hail of thorns says:
The next time you hit a creature with a ranged weapon attack before the spell ends
You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon’s range.
Although not specified, Volley should (probably) be a ranged weapon attack, as you are attacking with, well, your weapon. The only discrepancy I see between both texts is this lack of specifying that volley is a weapon attack, so it's the only uncertainty I can see.
Given the premise that hail of thorns does trigger, when does it trigger? In the first hit against a creature. The most logical (IMO) way to rule which creature was the first to be hit by Volley is to say they are being hit/targeted in the order you roll - so, the first creature you hit in your rolls will be the first creature your character hits, thus the one triggering hail of thorns.1
1 XGtE has a ruling about Simultaneous Effects (note: it's a suggestion, ultimately it's up to the DM how to rule simultaneous effects), which states
In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen.
The way I understand it, this must be done before actually resolving each effect, so you (the ranger) chooses which creature is the first you want to try to hit, roll the dice, and check if you hit. If you did, hail of thorns triggers. I think it would be unfair to choose which was the first hit after seeing what creatures were hit and what were not.