Indeed, besides pushing a new bitstream he also implemented difficulty >1 support, wow!
But why on earth does this warrant a breaking protocol change AT ALL? (Not to even mention that there was no prior notice.)
Well, maybe the old bitstream contained some critical flaw in the signcryption code that was fixed in the new one? I can't think of a different (rational) explanation. But OTOH it wouldn't be the first time for ET to act in a completely irrational way.
I suspect he accidentally released enough information for a clever hacker to bypass the signcryption. Been resisting the temptation to try.
So ... someone was hashing with his bitstream for free ... instead of ?
Sounds like a good reason to kill off everyone (without notification) mining using his bitstream until they work out what is happening ... NOT!