Giant Defy that seizes up every two weeks?

So I suppose it was my fault for not checking everything. 

A slight knee injury post-Dunwich, I've been off the bike for six weeks.  Yesterday, I go out to the garage, pump the tyres up, and oil the chain. 

This morning, out there at five AM for the cycle into work.  As I'm fitting the lights, I squeeze the brakes to check, and the left one is essentially loose, i.e. the lever goes all the way back to the bar, with no resistance.  I take a ride around the parking bit outside ours, and the rear wheel is binding to a the point where it feels like I'm in top, even though I'm in 'granny gear'.  

It's too dark at that time in the morning to see what the hell I'm doing, so the bike goes back into the garage, and I come back into bed for a couple of hours extra kip.  I'll go out later on to have a look. 

This is the second time that something like this has happened.  The first time, it was a brake cable that had seized up after a month during which I'd ridden my fixie. 

I don't get how this can happen to a bike costing north of twelve hundred quid.  It's an older Giant, so has mechanical disc brakes inIstead of hydraulic, but the heatwave of the past couple of months means that it hasn't exactly been sitting in a damp environment.  

It does seem that I can't leave this bike in the garage for more than a couple of days, without something seizing up. 

Any ideas?