Weird. I have never seen this in either of my '03's. I have had the SR5 since new, and only had to change one of these bulbs so far. Maybe there is an issue with the socket in the back of the reflector where the bulb plugs into? Also, have you, or a PO done any wiring/lighting upgrades or changes to the front lights. Maybe someone added aftermarket fogs or HIDs that were wired into the system harness?
its the socket. its happened to me multiple times and got to the point where it was frying bulbs after a week. replaced the sockets and its been problem free
I just changed out my license plate lights with LEDs and noticed the plastic housing was scorched...
Your housing might have arching going on causing it to get really hot. Look at the leads on the bulb base, looks to me like one side started to melt?