This is just a guess but I would blame the circuit board that the bulb plugs into. I kept having problems with my tailight bulbs burning out and that super annoying "lamp out" indicator thing on the dash dinging all the time. I found replacement circuit boards at o'reillys for like $18 each probably 4 months ago and haven't had an issue since knock on wood.
Why do I keep having to replace my reverse lights?
Just like the title says. One burned out a few months after I bought the truck. Same one burnt out maybe another 4-5 months later. Now no more than 2 months, both of my reverse lights are out! What gives??
