Transforming a layer in Photoshop creates a copy of the layer

I'm a regular user of Photoshop since the CS3 times, and this is the problem that haunts me from the beginning, regardless of the PS version, and the platform - when I do a transformation on a layer (Scale, Skew, Distort, Warp... but not Rotate), sometimes a copy of the layer is created and my transformation is done on that copy, not on the original layer. I say "sometimes" because I can't find any regularity in this behavior, sometimes it happens, sometimes it doesn't.

Weird thing is, I can't find anything about it on the Internet, like it's happening to no one but me. Anybody knows why is this happening and how can I prevent it?