My dad and a number of shops could not figure out why the brake lights would come on all by themselves. he ultimately added a brake pedal return spring (read... screen door spring) to pull the pedal up away from the master cyl and into the brake switch. When I saw the spring I said "That's not right" and removed it only to find the brake lights coming on at odd times. I contortioned my big old gut and all under the dash and found the adjuster bolt and moved the switch closer to the brake pedal when the pedal was in its natural resting position. No more ghosts in the car!
My best guess is that some aftermarket master cylinders have a deeper recess for the pushrod to go into and subsequently allow the brake pedal to sit lower that stock OR the switch adjuster just migrates over time OR replacement switches don't have the correct pushrod length!
WHO KNOWS!
On a side note if adjusting it doesn't fix it try another switch.
Good luck