J.,
I have read from several sources that this is indeed a phase, and that some toddlers do this to soothe themselves, even just to fall asleep in a perfectly calm scenario. The fact that he is consciously pausing to look for a good surface could be an indicator that this soothes him somehow. It could also be a way he communicates to you that he is angry or frustrated, since he doesn't understand emotions yet and couldn't communicate them if he did. It will help him if you provide the language for him for what he seems to be feeling, such as "You are frustrated!" or, "You want to show us that you're angry!" As his language develops, he'll be able to [yell] his feelings at you. He seems to be in control of the choice to do it, just maybe not in control of how HARD he does it. Hopefully, this last incident will help him learn about force! If he still does it at about 2 1/2 or gets more aggressive, you may want to talk to his pediatrician about testing him for a sensory disorder, but at this age, its perfectly normal!