What is the warhead on these things?
Answering my own question it appears to be a PTAB bomb: https://en.wikipedia.org/wiki/PTAB_(bomb)
I don’t understand how this video resulted in a kill though; I don’t think the turrent-top armor would be a “weak spot” that a PTAB could exploit. Maybe the issue was the open hatch?
Well how much armor do you think an open hatch provides?
The open hatch almost definitely lead to the kill. You can overpressure loaded or stored shells with less than the .62kg explosive charge in the bomb. There’s a small chance though that it penetrated; T90a’s turret top armor has at most 200mm of armor, of which 90mm is steel; PTAB have been known to penetrate 90mm of steel and the rest of the armor composite does not resist HEAT rounds.
Most of the time it is an RPG warhead