Occasionally when I enter combat with enemies they will throw one or two explosives at me whether they are molotov cocktails or frag grenades. I recently fought the Raiders in Hangman's Hallway and after shooting one of them dead I proceeded to hurl about 4 cocktails into their base. They returned fire by first throwing a molotov at me, then a frag grenade, followed by another cocktail in between their gunfire.
I don't know if my thrown explosives made them more willing to throw theirs or if it was because I was on Very Hard difficulty but I found their seemingly irregular bout of throwing explosives strange in comparison to other enemies.