DIAGNOSIS: Water or moisture has gotten inside the handle and is shorting the power button circuit. This is almost always caused by rinsing the handle directly under the faucet.
FIX: Never rinse the handle where the button is. Only rinse the brush head. To fix the current issue, remove the brush head, shake out any water from the handle, and let it dry COMPLETELY in a well-ventilated area (not on the charger) for 2-3 days.
IF THAT DIDN'T WORK: If the phantom activation continues after thorough drying, the internal switch is compromised. The only reliable fix is to replace the handle. This model is not designed to be waterproof at the button assembly.
HEADS UP: Store the handle upright, not lying on its side, to prevent water from pooling near the button. This is a design flaw in many basic Oral-B models.