Unlike the usual dispenser behavior where dispensers fire when powered and given a REDSTONE update, dispensers fire when given any block update (while powered).
Demonstrated here: http://www.youtube.com/watch?v=qyzFkmp3G3c&feature=youtu.be
As seen above – Lava gives a block update to dispenser, causing an infinite loop of updating and firing to occur.
Duplicate of MC-10948, please use the search function to see if your bug has already been submitted. Currently over 53% of tickets are being closed as duplicate.