I've seen this before and I had it explained to my by Dave Pincheck at Extron. So Dave if you are out there, you may want to pipe in....
From what I understand, the subtitles are added by the DVD player and are not part of the original movie content. So, when the text is inserted by the the player it doesn't follow the proper 3/2 pulldown pattern and it usually tears. The fact that the text comes and goes at random times can actually throw some de-interlacers out of film detection.
Dave, anything else?.....