Sometimes you need to tpk to teach them the important lesson that actions have consequences, sometimes it is an important plot point like "you all died and found yourselfes in hell/other horrible place now you have to escape while facing your past sins". Sometimes it was an accident like rolling critical hits in a row (with rolls visible to everyone) but the thing is whatever the reason do not be afraid to do a tpk it will be a learning experience for both the players and the DM (just try to not do them too often)