It is sort of random, at least in my experience. If the drop in connection between XBMC and the server hosting the file is just for a moment, the video normally seems to resume happily enough. If (for example) my internet actually goes down, then by the time sync is re-achieved, the movie restarts completey.
Hmmm, I wonder if a change in IP address could cause this? If you want to double check, load up
http://whatismyip.com on your phone/tablet (assuming it is on the same WiFi network) at the start of a film. If the film pauses then restarts completely, reload the website and see if your IP address has changed. I could see how an IP address change might force a restart.
Ah, a new message has been posted. I could be repeating what others have said.