This can be a problem in the older Foresters and although you have it working now be prepared for it to happen again. The calipers on a lot of the older cars are prone to seizing I think and the only way to deal with it properly, short of buying a new one, is to strip and rebuild with new seals (and pistons too if you have the money, plated reasonable, SS a good bit more costly but it's releative I guess). There is lots of info to be found online on how to do this. I had a recurring issue on the drivers rear and despite changing the pads and greasing the outer part of the piston, under the dust seal repeatedly, it came back and eventually I bought a used part which was stripped and rebuilt it prior to getting it fitted.
The front calipers are prone to this as well where one of the pistons seizes, dramatically reduing efficiency. When this happens on both sides, as well as the rears joing in, it becomes a liability in an emergency although daily driving seems alright. When I upgraded the front to 4 pot/294mm discs I discovered that both fronts had one piston siezed and one of the others, on the drivers side I think, was stiff too. A friend who has a 2000 S turbo on factory brakes doesn't have this problem, I nearly faceplanted in the car park the first time I drove it so if all the calipers are working, it seems these brakes are ok. Some guys I know over here track their cars and a couple of them are running stock brakes albeit with HD pads so I guess they must be ok even if the WRX is somewhat lighter.
