My husband and I were married just short of a year when I cried during/after orgasm. It was scary at first, I almost strted to laugh, my reaction was "what the hell is happening to me?" But the second time I felt it coming sooner. We have gone through a very rough year, and to top it off I had started seeing someone around the time the crying started. I never slept with the other person, but to this day I feel I was more in love with him.
I think it is something that is different for everyone. It stems from a release of emotions that have been hidden. For some people it is the overwhelming feeling of love for thier partner that is not seen day to day, but then felt when making love and hormones surge causing crying.For others it could be sadness, stress, guilt, PMS, or a mix of emotions.
Hope this helps