Seized caliper will be the piston seal. Will need to pull the caliper off and clean it out, replace the seal and check the bore for any scoring.

The slide pins won't cause the caliper to seize on. The brakes are applied by hydraulic force, as you all know. The force pushes the piston against the pad and in the case of a floating caliper liek yours, it will "squeeze" the disk against the opposing pad.

The thing which cause the piston to retract is the seal, this pulls the piston back off and relieves any pressure.

So rebuild the calipers and give it a good fluid flush.