What would cause a set of alkaline batteries to get VERY hot?

[Jeff Haas]

I have a few 12V battery packs, made up of AA batteries in a standard battery holder, that I use to power some of my Prop-1s and Prop-2s.  I was testing something and went to move one of these and it was HOT.  Every single alkaline battery was hot and the plastic on the ends was starting to melt.  I pulled it out of the Prop-1 and tested it, the Prop-1 is fine.  Nothing else got damaged other than the batteries and their holder.

What would cause a bunch of batteries to do this?  A short somewhere?

[bsnut]

A short in the battery pack or on the connection end to the Prop-1.