To explain further every back has some maximum exposure length.
Some backs have a "hard" cut off meaning they turn off automatically when they hit a certain length.
Other backs have a "soft" cutoff meaning you can exposure any arbitrary length with the understanding that some point the quality will become unacceptable.
Here is a story that contains some industry background and technical information about long exposure in medium format. It's a biased article (I wrote it to explain the backstory to the iq260's long exposure capability, and my company does not sell Hasselblad backs) but should still prove an interesting read if you want to understand the technical reasons or the limit you've experienced.
IQ260 Story