Implying that it is MCP1703, it has overcurrent and overtemperature protection, in my opinion the most likely scenario for failure is input overvoltage. MCP1703 would be 16V absolute maximum. The layout is quite bad, look at burnt regulator: it's input bypass cap is who-knows-where, maybe quite far, thin long trace (read - significant inductance). Output cap C6 is not sufficient for stability at all conditions. I perfectly imagine, that regulator input might experience voltage transients in some situations. Long traces and ceramic caps are a receipt for ringing. Can be risky if margin is low. You said it is up to 15V? So almost no margin. It might not be a coincidence, that the failed regulator is the one with bad layout. The other one in the picture seems to have better physical arrangement.
Generally speaking, proper bypassing and choosing a part with higher input voltage (more margin) would likely prevent this from happening again. Of course, very high working temperature might also cause failure long term. So check that it is not running near limits.