Rear brakes locking up?

I noticed that my rear brakes on my car will lock up when on loose dirt. I had it happen to me again today it wasn't the first time. I was driving down my drive way foot off the gas just at idle. I went to apply the break and didn't apply it very hard just softly to slow down. The rear brakes locked up on the gravel drive way. I am wondering why they would do this i believe i have abs and the light is not on. What could be causing this? I haven't pulled the rear tires off to look at the drum brakes yet. Any help would be great, i don't want brake failure to cause an accident. thanks

Could be a leaking wheel cylinder. Pull the drums & check for brake fluid on the backing plate.

I have a 99 that sometimes the rear brakes appear to "grab" and make a noise, when applying them in the driveway. I have looked at the shoes and nothing looks wrong. Never noticed it on the road., so I figured it is just normal.

I just got done talking to a guy at our local dealer and he told me that the top of the brake pad on those cars catches and causes it to grab hard and seem to lock up. To fix this they simply grind off the pad at the top. i will post a pic so you can see what i am talking about.

http://img126.imageshack.us/img126/1636/brakedrumdk1.pnghttp://www.automotiveforums.com/vbulletin/%5BURL=http://img126.imageshack.us/my.php?image=brakedrumdk1.png%5D%5BIMG%5Dhttp://img126.imageshack.us/img126/1636/brakedrumdk1.th.png%5B/IMG%5D%5B/URL%5D I know this looks bad but it is the best i could do on the computer with short notice and no time.

Buick24's diagram illustrates why the primary shoes on drum brakes USED TO be shorter, back when drum brakes were a lot more common on all four wheels. It seems that lately the accountants have won out and are forcing the use of the same part (less inventory) for both the primary and secondary shoes on these light- and medium-weight vehicles with rear drums (it's a self-applying brake design, so the leading shoe is the primary). Current vehicles which are larger (and use the larger Morraine drum brakes on the rear axle) still have a shorter primary shoe:

http://72.19.213.157/files/MorraineRightRear.jpg

thanks for explaining that blue bowtie.