Rolled backwards on a hill, ABS sound?

I was in a parking lot that had a very step exit onto the street and when I took my foot off the brake to accelerate the truck rolled backwards a little before I pushed on the gas and it sounded like the ABS came on. Has this happened to anyone before and can some one tell me what the truck was doing? Tires were not spinning while leaving the incline to the street. The truck is a 2011 DC OR model.

 

Is it supposed to activate on the pavement?

 

Cool. I just never had the truck roll back on me before before I started off. The sound bothered me since it has never happened before. Thought I screwed something up.

 

I think the truck is smarter than I am LOL.

 

Thanks for the info Whippersnapper02. Do you know what is engaged when this happens?

 

Thanks again.