D600 inadvertent exposure change

Last Sunday I shot an Under 11s soccer match. Before leaving home I set my D600 and 80-200 f/2.8 thus:

1/1000s; F/4; Auto ISO (max 3200); Centre-weighted metering; CL @ 4fps; Dynamic AF 9 points; back-button focusing. I took about 400 shots during the game.

When post-processing in LR4 I noticed that the shutter speed of some mid-period shots had increased to 1/1500s before reverting to the 1/1000s I had originally set. I did not intentionally change any of the settings.

Two questions arise ...

1) How do you think it happened?

2) How can I prevent any re-occurrence?