Combined gas law says: (P1*V1)/T1 = Constant or = (P2*V2)/T2
Adding on Keith feedback, and looking at the formula above, when theres excess (increased) pressure, theres an increase in Temperature due to the faster moving molecules (assuming volume remains constant). To compensate for that engineers introduced various damping mechanisms/devices on pressurized systems (hydraulic & fuel pumps).
One of those mechanisms is the thermal relief valve.