MediaTek devices use different USB connection modes depending on their power state. When you first power on a device or connect it while it is off, the system briefly enters or BROM mode before starting the normal Android OS.
A faulty USB cable is the #1 reason for BROM driver failures. Use a (not a charge-only cable). Avoid front-panel PC ports; use rear USB 2.0 ports directly on the motherboard. driver mtk brom vcom