Cell phone stays on after connection with Bluetooth

[New Daddy]

My XV 6700 stays on for a few minutes after it is successfully connected to the Bluetooth headset in my car.
Even if I press the power button to turn it off, it defies my intention and the power turns back on immediately.

I wouldn't have a problem with this otherwise, but because the phone is on, keys tend to be pressed and do things I don't intend (like butt-calling).
Why does engagement with the Bluetooth headset make the phone powered on for the first few minutes?