i have never seen anyone use a blowoff tube and an airlock at the same time. Of course Im no expert, seems odd to me though. From what i can see it didnt utilize the blowoff because of the airlock? Or was the cap not on tight enough?
You don't have a big enough blow off tube: I'd get one at least the size of the cylinder of your airlock. Stick it directly on the airlock. That will let more air escape and have less chance of the airlock exploding for an even larger mess!