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It's out of power. Bluetooth headphones are battery powered. If the battery is out of power or damaged, it will often disconnect automatically.
The interference source is too strong. Bluetooth headset is working at 2.4g frequency, this frequency also has wifi router, cordless phone and 2.4g mouse. These are relatively strong sources of interference.
The headphones are of poor quality. In particular, refurbished headphones, just bought normal, with a period of time out of the problem.
Bluetooth headset compatibility problem.