I've had this happen a few times, it happens on the first brake use of the day. What it does is grab let go then grab again after that it is fine.

 

Moisture absorbed during the night. After applying a couple of times they dry out.

 

I have the same problem, it started happening about two weeks ago for no reason that I can put my finger on. The back tire'll grab for just a sec, let go and not cause any more problems. I just feel wierd pulling up to a stop sign at work, and the back tires lock up and cause a ruckus. Makes me feel like I'm going too fast. And it's very touchy too, doesn't take hardly any pressure on the brake to set it off...

Sorry I don't have any info for you, just wanted to let you are not alone.

 

Just thought of another cause, if you have drum brakes, check the wheel cylinder for leaks. A little brake fluid will make them grab. A lot will make them go away.

 

Get your trucks to the dealership. Mine was doing the same. Ford deemed my vehicle unsafe. Saying that the rear calipers were failing. And come to find out the rear calipers are on a national back order. MUST BE HAPPENING ALL OVER.

 

If you have drums, it's water inside of them. It will dry out after a few stops. Check to see that your plugs are still there...