Because they're the spawn of the devil
Seriously though, I have a suspicion it has something to do with the fact that Clownfish and Condylactis anemones aren't a natural pairing. Perhaps the anemone is recating to the clown bothering it by releasing tentacles or perhaps the clown is bothered that the anemone isn't be a very hospitable host and getting rough with the anemone as a result. While I've never kept the two together I've seen several "experts" suggest that the pairing of Condylactis with Clowns usually results in the death of one or the other.