(caveat: I'm still very new to 5e monster design, so apply salt as needed)
Given how you've narrated Headbutt, it might make more sense as a bonus action. It would be a trait, reading something like:
"Headbutt. When the cave troll successfully hits the same target with two claw attacks, it can make a headbutt attack against that target as a bonus action. It may smash its forehead...."
(I only mention this because I've been looking a lot at the Lion statblock last day or two; and you've framed this attack similarly to how a lion's Pounce trait works, as a follow-up to a successful normal attack.)