Jamieson, Fausset and Brown on Exodus 22:6

6. If fire break out, and catch in thorns--This refers to the common practice in the East of setting fire to the dry grass before the fall of the autumnal rains, which prevents the ravages of vermin, and is considered a good preparation of the ground for the next crop. The very parched state of the herbage and the long droughts of summer, make the kindling of a fire an operation often dangerous, and always requiring caution from its liability to spread rapidly.

stacks--or as it is rendered "shocks" (Jud 15:5; Job 5:26), means simply a bundle of loose sheaves.