Has the hose worn through where it was rubbing?
Were the brakes ok before the MoT?
When the caliper was removed, did one of the metal hose ends get 'tugged' (brake calipers are quite heavy and if they dangle it can damage the ends)?

I'd have a good look round for tiny fluid leaks. I guess it's also possible that during an MoT the testers will give the brakes a bit of a work out which might finish off a weakness, whereas if you tend to drive very sparingly on the brakes it might not have shown up before.

Do you have drum brakes on the back? If so the slave cylinders can go when they're disturbed or suddenly worked harder than normal. I changed the rear brake shoes on mine a while back and there was a trace of a leak inside one drum, but by the time I'd fitted the shoes it p**sing out! Then the second drum, which hadn't been leaking, also started weeping the moment I started work on that side. I replaced both rear slave cylinders and the brake fluid and the brakes worked better than ever.

Hope it helps.