Keep the warning's own wording
Google documents a Low power or g.co/cast/power screen for supported Chromecast with Google TV and Google TV Streamer devices when power is insufficient. An explicit device warning deserves a different investigation from an in-player buffering indicator. Record the model and exact screen rather than translating both into the internet is slow.
A restart can also return the device to its startup sequence. Note whether a boot logo appears, the player itself reloads or only the picture pauses. These stages help distinguish a whole-device event from a media action. A screenshot of a black frame alone may omit the warning or boot sequence that makes the event understandable.
A hypothetical television USB connection
Suppose a viewer powers a supported streaming device from the TV's USB port and sees the documented low-power screen. Google's guidance says to check the supplied adapter, reliable wall outlet and original cable. The relevant next step is the supported power arrangement, not purchasing another viewing term or resetting the player's account.
Check that the cable is fully connected and that an intervening hub or switch is not creating an unsupported power arrangement. Use the exact model's official replacement requirements if a component is missing. A connector fitting physically does not establish that an arbitrary charger and cable provide the required supported supply.
Do not repeatedly factory-reset the device to investigate the warning. Preserve installed-player and account information first. Also distinguish disconnecting HDMI from disconnecting power: Google's reboot guidance notes that removing the HDMI connection alone does not reboot a device that remains powered.
Repeat the original viewing sequence
After following the model's documented guidance, start the device normally and repeat a short authorised viewing session. Record whether the warning returns and whether a whole-device restart still occurs. A working menu is useful progress, but the original symptom happened during the intended session and should be checked there.
If the warning persists with the supported arrangement, contact the device's official support with the model, message and power-chain details. Keep the viewing account separate from the hardware enquiry. The result should identify whether the documented device warning was resolved, without claiming an unperformed power test or a universal explanation for every reboot.
Questions answered
Does unplugging HDMI always reboot a Chromecast?
No. Google notes that a device remaining connected to power is not rebooted merely by removing HDMI.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.