I'm no expert but from what I've read on this forum and others, the relief valve can 'break' and start leaking. Obviously, it should only leak when you want it to. Replacing it should be in order. Make sure you do this at a time when no one needs hot water for a while in case you have a problem. Shut off the unit, shut off the feed line (or the house main if necessary), and drain slowly at first. The water will be HOT-HOT-HOT so do use rubber gloves. Get a hose to hook up to the drain valve. AND, get a WET/DRY VACUME, and have a plan in case the whole thing blow! Hopefully, you have a tub nearby or an outside door where you can dump the water. DO take out the dust filter from the VAC before you start! The rubber gloves will come in handy if you need to manually (literally) plug up the tank. The 2nd person helping you drain the tank (you WILL have someone else around, won't you) can vacume up while you plug the tank. Then, replace the pressure valve AND if you can, replace the drain valve with a metal one if you don't already have one. The plastic ones fail quickly especially PROBABLE after they're used (as in your case). There is a conversion kit for this job. Then, go have lunch and replace the valves. Then have your helper around when you fill the tank up. Do everything in the REVERSE order, very slowly.