First: In Tetris nuking 4 rows at once gives you a greater score than nuking 1 row 4 times. (Same goes for 2 and 3 rows). As the guy learned the game he would have gone for optimizing his score rather than number of lines nuked. As he got better the habit would have stuck even though it wasn't strictly necessary to play it that way. The way to nuke 4 rows at once is to build up everything in 4 or more rows minus the slot that a 4x1 can fit into vertically (the longest piece in Tetris).

Second: Leaving a gap on the right or left rather than somewhere in the middle is the easiest way to accomplish this. If you can't see that, I'm sorry for you.

Third: This guy chose the right.