Basically, always remember to never plug up your TPR valve. If it is leaking water, it's likely because it has too much water pressure or too much heat. It's made to release excess pressure so that your water heater doesn't explode.
Here's a video of one exploding to give you an idea of the power released during a water heater explosion.
Video description:Video of a 50 gallon water heater exploding. There was a pressure gage on the top of the tank. We used a spotting scope to read it from 150 yards away. Minutes before it exploded the pressure was 330 psi. Total time was between 3 and 4 hours.