Why do my snowboard bindings loosen so easily?

Every time I go down a slope, my bindings loosen. After only a few runs, the bindings are loose enough that they wobble if I turn my toes in or out. I have to stop and tighten them every few runs (unless I get lazy and just go with loose bindings... which happens a lot because after the first few times, it gets really annoying to tighten). So, why is this happening? Is it something I can fix (my snowboarding technique, how tightly I'm putting my bindings, etc) or is something wrong with my bindings?