When chests drop, right-click them and one of your options will be "disarm."
When your disarm skill is low you will often set off the traps by trying to disarm them. This will definitely happen to you if you have not been disarming all your chests.
Also, there was a problem on release with the disarm traps skill not skilling up properly. I think it's been fixed, but a lot of people are in the same boat with you -- they've levelled but haven't been able to level that skill proportionately, so don't despair.
I'm still working on raising mine. Last night of roughly 30 chests I tried to disarm I think I was successful on perhaps 4 of them. Stick with it...this will be a very useful skill later as the traps on chests are powerful enough to kill your whole group.
Hope that helps.