deanshark

It definitely looks like you got some water in there, which might have froze with all the cold weather we all have been experiencing lately.
And we all know (thinking waaayyyy back to school days) water expands when it freezes. But if that's the case, why would it only happen on one brake and not all, and why would it not be locked when you go in reverse? I don't think that would be the cause of it locking up.
Seeing how you got the car home and can work on it, (weather permitting) your best bet would be to pull the brakes and rotor from that side to see exactly what's going on in there. I highly doubt you would need new brake lines unless they are rusted. I looked into changing some of mine on my 94, when it was rotted, and realized there are soooo many brake lines it wasn't worth the value of the car and some are a big PITA to change. But you should definitely change out that fluid.
I never asked before but do you regularly use the E-brake when you park your car? Has the car seen alot of snow and salt conditions?

lamb03

yeah you’re right, that freezing weather here was pretty bad so i shouldn’t be surprised it damaged multiple things at once but i always apply my e brake before shifting to park because i thought it would save me from having issues with the brake lol
Rarely drive it in bad conditions on salted roads, i used to have a major oil leak and i think that’s what kept my underside in great condition rust free. But I definitely need to figure out how that much water got in the brake system all of a sudden

Losiracer2

make sure one of the e brake shoes inside the drum didn't come unglued from the backing plate and is spinning around inside the rotor hat. I would take everything apart and inspect. Also, check for collapsed brake rubber hoses, that can happen with age and then your caliper will randomly hydraulically lock if the pressure can't be released.