What can be happening here:
- badly designed output Audio section shorting power in some rare circumstances, might be as weird as mechanically stressed audio jack touching traces underneath it.
- software glitch around audio power enable routine enabling/driving that transistor hundred/thousand of times per second (pwm) in some circumstances, keeping it in the linear region
- badly designed under powered mosfet driver, either voltage too low or not enough current keeping it in the linear region
You do not cool power rail switches like this one, they arent meant to dissipate any meaningful power when designet properly.
It could a whole slew of problems and it's impossible to tell without measuring. But, I would guess that gate voltage is not far enough above the threshold to drop the Rds(on) to it's low loss on-state.
We don't know the voltage on the rail, so we can't say exactly what it is, but the likely candidates are an SOA violation because they're switching it absurdly slowly, or the gate drive is below a reasonable threshold.
Not just a cooling issue. It's a MOSFET thermal runaway, as it is being driven to its limit, higher temperature => higher resistance => higher temperature. It can be stopped by cooling it, but the bigger picture is that it is either being driven improperly, or cheap, low power components are used to cut costs without adequate a safety margin.
So it seems like the problem is a cooling issue, which doesn't surprise me. "Ultrabooks" often seem to sacrifice decent cooling for the sake of thickness, and I think it is because most people really do not tax their hardware all that much. If you do - and you do not have a proper workstation-class laptop - you're likely to run into similar issues.
My Thinkpad X280 suffers from poor cooling that quickly leads to throttling, even undervolted. My old work A485 wasn't that great, either.