haha i hate when that happens. usaully i can feel air inside me and i kina know a head of time when its going to happen. a little advise dont laugh unless she laughs first my bf always tries not to laugh but when i laugh hes knows its cool to laugh too. and it can be very embarassing.
It is funny when this happens, and embarassing. But it's both of your fault. Sometimes when switching positions air gets trapped inside. This mostly occurs when doing "doggy style". If it bothers you then don't do that position. But it's just a part of life and completely normal.
Your wife probably was embarrassed.
It is usually just air that gets trapped in the vagina with thrusting. So you can tell her it isn't her fault, it's yours. ;) (just kidding)