MacBook Air powered down itself

As already mentioned by Zedainder, while modern Macs are generally very reliable, there are still a few scenarios where macOS may intentionally shut the system down. Common causes include a thermal protection event, a low-level kernel panic, a brief power interruption at the USB-C port or power adapter, or macOS triggering a protective shutdown after a hardware or driver fault. These events can occur even when the battery health appears normal and the Mac is plugged in.

I suggest to help narrow this down, the best next step is to review the Mac’s system logs around the time the shutdown occurred. To do so, please open the Console app, and then look under Crash Reports, Log Reports, and the system.log for messages mentioning “shutdown cause,” “kernel panic,” “thermal,” or “Previous shutdown cause.”

In addition, if you previously enabled "Share Mac Analytics" in System Settings > Privacy & Security > Analytics & Improvements, you can review the "Mac Analytics Data" reports in the Console.

To make reviewing the logs easier, I suggest using some free utilities from The Eclectic Light Company.

In the meantime, I’d suggest that you confirm that you’re using an Apple or known-good power adapter and cable, ensuring the Mac has adequate ventilation, and checking whether this happens again. If the logs indicate a hardware-related shutdown or the behavior repeats, the next step would be Apple Diagnostics or an Apple Support evaluation.

You’re absolutely right that under normal circumstances, a brief power interruption should not shut the Mac down because the battery takes over instantly. However, if macOS detects an abnormal condition at a very low level (for example, a power negotiation fault on USB-C, a transient hardware fault, or a protection trigger in the power management controller), the system can perform an immediate safety shutdown rather than attempting to continue running on battery. When this happens cleanly, it often does not generate a crash report or panic log.

The external USB-C disk you mentioned is also worth noting. USB-C ports handle power, data, and device negotiation simultaneously, and in rare cases a misbehaving peripheral, cable, or sudden disconnect can cause a brief power or controller fault that leads to a protective shutdown. This doesn’t mean the disk or the Mac is defective—just that the system chose the safest possible response at the time.

Regarding logs, what you’re seeing is expected. Some low-level shutdown causes are recorded only in volatile logs, which may roll over or reset after a restart, especially if the shutdown occurred before midnight or did not qualify as a crash. Thermal shutdowns are also logged only when sustained overheating is detected; a transient sensor anomaly or controller-triggered shutdown may leave no clear trace.

Given that this has happened only once, the Mac has been stationary, and there are no recurring symptoms or logs, this points strongly toward a one-off hardware or power-management edge case rather than an underlying problem.

Dannymac22 wrote:

Maybe some short as it was being unplugged?

Maybe. Anything is possible. At least now, you know to review system logs should this same thing occurs in the future.

It’s not uncommon for a MacBook to power off after a long period of no use. It’s just a way to preserve power and prolong battery health. Is this this case?