Q: Built in camera issue fixed, why?

I have a mid 2012 Macbook Pro with retina display running 10.11.3. The built in camera stopped working and none of the troubleshooting tips in 'How to Troubleshoot iSight - Apple Support' worked. I checked System Report/Hardware/USB and the FaceTime HD Camera was not listed under USB 2.0 BUS. I reset the SMC but the issue persisted in both Skype and FaceTime. I ended up wiping my hard drive and installing a fresh copy of El Capitan from a bootable USB, not a backup, but this didn't work either, the camera still didn't show up under system report.

I figured this isolated the problem as a hardware issue and scheduled an appointment with Apple. I ended up re-installign Skype on the fresh OS X install to use as voice chat, and when it ran though the initial setup steps where it allows you to select which microphone and camera you want to use, the camera turned on. The camera works in FaceTime as well and now shows up in system information.

Does anyone have any idea what went wrong and what fixed the problem? I need my camera for a Skype interview and I want to make sure this issue won't happen again. I'm not sure if this is related, but my boot times skyrocketed on the same day the camera issue started. The computer would stutter on shutdown, and after restarting the screen would turn on but remain black for at least ten minutes before booting regularly into OS X. The fresh install fixed the boot time issue.

Thanks!