Are you sure it is actually crying? Toward the end of most movies, both in the theater and on planes, my eyes tear excessively. I would always explain to my date/friends/traveling companion that this always happens to me before the movie started to avoid being slightly embarrassed.

On a routine eye exam last year my ophthamologist asked if I ever experienced periods of excessive tearing and I mentioned these examples. She asked if I had arthritis. I have traumatic arthritis from an injury. She said it is quite common in people who have arthritis. The other reason is from eye strain, staring at an object at a distance for a long period of time, in a dry heavily air conditioned space (like a plane or movie theater).