Why are my brakes locking up when I step on the bake the pads tighten around the rotor and stay?

There are a few possibilities. They could be wrong pads. You may have gotten hardware mixed up and used a bolt that is toolong and is locking up against rotor. The pas retention spring may be missing or pad incorrectly installed.

Let me add to this list
Bad calipers
frozen caliper slides
bad flex hoses
bad master cylinder
fluid contamination
incorrect push rod adjustment

Needless to say, you probably need to have it inspected by a professional.