Water – Electric water heater T&P popping off

Expansion tanks can fail over time. There should be a valve stem like you have on a car tire to add air to the tank. Shutoff the water supply, open a faucet to release any pressure from the water on the tank, and pump up the tank to about 60psi (even better is to fill to your average water pressure at your home, but 60psi is typical). If the tank doesn't hold the pressure with an open faucet, then the internal bladder has ruptured and the expansion tank needs to be replaced. They are usually threaded, so you can unscrew the old tank, apply some plumbers dope or Teflon tape to the threads, and screw on a new tank (pressurize it first).

Your expansion tank has likely failed or isn't large enough for your hot water tank. Expansion tanks have a bladder in the middle that ensures it is half full of air and needs to be periodically checked to be sure it's properly pressurized. These bladders eventually fail, allowing water to enter the air side, which eliminates the ability to absorb the expanding water.

Replacing the expansion tank may be a simple process if it's just screwed on. With the water shutoff, you unscrew the existing tank, pressurize a new tank, apply some plumbing dope to the threads, and screw the new tank on.