He got chin checked. This is not his fault but the fault of his horrible team/trainer.
He was chin checked during the Chazz fight, even forum tards realized that Mitchell have an issue with leaving himself too open. Professional Trainers should have caught on to what he did wrong against Chazz and fixed it asap.
The reports coming from his camp was that they didn't believe Banks was a big puncher, so they don't feel like Seth needed to be defensive. They wanted to put forth a giant first round knock out to impress on HBO.
Honestly, their theory is sound I guess. Banks is NOT a big puncher, and Seth did need to impress on HBO. Too bad for them, Banks was ultra motivated that night because of Emmanuel.
I really believe it also had a lot to do with Emmanuel's passing and the motivation it gave Banks.
Look at Wlad-Wach, he haven't fought like this since his 3 knock outs.