Truck swerving on hard braking. Thoughts?

[macmann]

My truck has 147k miles. When I brake hard, the truck swerves to the right. I checked brake pad thickness on both front wheels. Seems to be fairly even on both sides. Any other suggestions as what could be causing this swerving?

 

[Sandroad]

Recently stuck caliper piston(s)
Contaminated brake pads
Pads broken or stuck
Bad rotor turning job
Rotor damaged
Air in brake lines
Damaged brake hose
Rear brake problem (see above)