Tried to reverse car out of garage yesterday and the rear calipers had seized, the car had only been there a week and I leave it with the handbrake off! I rocked it back and forward for a bit and they free'd up eventually. Id washed the car before I put it away last week so there was rust on the discs but even so why should this happen?Any ideas?

Like you said, you put it away wet!!
Thats a big 'no-no'!
if you wash the car, just drive round the block or something before you put it away!

Fair one, cheers.