Wasteland Strangler triggering multiple times

wereotter

Playing in the current Oath of the Gatewatch event, and my board state has Prized Amalgam in the first creature slot, and Gisa & Geralf in the second.

My opponent cast Wasteland Strangler and processed to give my Prized Amalgam -3/-3 and killed it, so far no problem. But then without casting another copy, he triggered again and gave Gisa and Gerald -3/-3 as soon as Prized Amalgam died. There was no trigger for this ability.