If a demon cat's pathfinding decides to end one block below the height it is currently at, the demon cat will reach the position, but not drop down, and will twitch like crazy (in an attempt to reach the position one block down, where it was attempting to go).

A slightly smaller bounding box may fix this? Not sure, but normal ocelots don't have this issue, so probably something to do with either collision bounding box or pathfinding.

Edit: this is easy to reproduce; just grab a handful of cat eggs and spawn them all on the peaks of trees.