Why do I tighten up when big stacked?

[Foolgizaki]

I have been playing poker for several years, and have experienced the "big stack blow-up" too many times to count, mostly because I loosen up and try to use my large stack to pressure the rest of the table. I have noticed that recently, when I get big stacked in a tournament or even double or triple up in a cash game, I tighten up and become weary of bullying off my chips, or even getting involved in any coin-flips. I even become scared to play moderately-big hands like AK or 88. I go into rock mode and eventually miss solid money-making opportunities. Any tips as to how I can avoid this in future sessions? Thanks!