Thanks l_bRAD, rhenning, Booger1, I'll take better care with the next tube. (And let some air out of the front tube before I end up with another surprise!)
Just because the tire says it CAN go to 90, does not mean it SHOULD go to 90. It's a common misconception.
Yep - I read online somewhere that maximums are understated, and that good practice was to inflate to the stated max. I failed to think about the fact that these are modern tires on old rims (and also that I shouldn't believe everything I read online!). Ah well, live and learn!