Probably just a short circuit or something. My understanding is that it wouldn't happen in flight, but being low to the ground it'll activate the ability for the slides to deploy. It's not too dissimilar to the Asiana Airbus slide deployment in flight a couple of months ago — theoretically a plug type door can't open in flight, but when the aircraft is on final and pressure is relatively equalized, it's technically possible. Given the hinge assist mechanism on the Airbus along with the low altitude, it made it possible for the door to open and the slide to deploy. With the 76, the slides probably have the ability to activate below 200AGL or something, and if that slide short circuited, it could have caused it to deploy since they're held externally. Just a guess though.