To start; to cause, especially used for unstable situations that may magnify if disturbed
Be cautious talking about religion or politics, lest you touch off an argument.
If something touches off a situation or series of events, it causes it to start happening. Is the massacre likely to touch off a new round of violence? = spark off
[ 't&ch ] (verb.) 14th century. Middle English, from Old French tuchier, from Vulgar Latin toccare to knock, strike a bell, touch, probably of imitative origin.