Teemo Shrooms

It is the ward rubble that kills the shrooms

Yeah it's not exactly the rubble itself, but the brief moment when it becomes rubble. It also affects other champions.

http://boards.na.leagueoflegends.com/en/c/bug-report/h9IsovKW-gameplay-dying-wards-activate-shrooms

Either way, seems like a bug to me. I have also experienced this. I lay down a shroom in an empty bush and then a few seconds later it explodes. Needs to be fixed.

I can confirm this, it happened at least 8 times in my last game.

Moreover, Mushroom ticks no longer trigger Spellthief's Edge like it used to as they still should.