It's not uncommon, I've had this happen too in the few MTT's I've played. Basically when you start thinking "heh this is easy" and "all these guys are such fish rofl" then it's over for you. I dunno what you can do about it except remind yourself of this bad trend when you notice you start getting too cocky again. I've had this in ring too at the beginning whenever I got a bigstack. Now it doesn't happen anymore.. I guess it's an experience thing?