The t&p valve will leak from an increase in temperature or pressure which both cause pressure to increase. Sometimes they just go bad. A few bucks to replace would be the first start.
If you had a pressure reducing valve added to your water line coming into the house, then you need an expansion tank to go with it.
Check the temperature of the water. A runaway thermostat could be overheating the water. Thermostats are cheap too. Post back with more details about when it does it, how often, and if you or the city have done any work to the plumbing or water mains.