I do this a lot: I gamble with my opening moves, and overextend myself with whatever I try and loose to a more conservative player than my self. I assume it has something to do with a desire to avoid a prolonged game that drags out. I want to do the unexpected and then steamroll my opponent. It rarely works.
Today I played two games as Terran against different Zerg players. I didn't pick Terran, I got that race through picking random and ending up as Terran. I wanted to contain the Zerg and push in on his base as soon as I got my first tank. In theory this should work, but the first one simply ran his Zerglings past my forward bunker and killed my base, and the other teched straight to Mutalisks, which then raped my base.
I could have been more conservative and built some defenses, but I wanted to believe that I was in control. I obviously was not. Maybe part of the problem was that containing an opponent does not really put pressure on him the same way attacking him directly does.
In another game I ended up as Zerg on the map Hunters against a Protoss player. I thought I could hide an expansion at one of the other starting points and then out-produce him in units. The problem here was my assumption was wrong and he did discover my expansions and killed it off. Then I was way behind.
