Perhaps some application you have installed and has requested permissions to modify system parameters, and only activate bluetooth when needed, or it can also be a malware that has sneaked into your system through an app and activate bluetooth and this receptive to the order of a nearby attacker and want to hack your phone via bluetooth, but you can restore the phone and if it continues with that behaviour you will have to do a hard reset

Well it depends if you have an iPhone check if it is like this

If it is, it is temporarily turned off and will turn back on. To avoid this you have to turn it off in the settings directly so it doesn't turn back on and the same with wifi.

If you are on an Android well take it to a mechanic or update it or reboot it can also help but better the mechanic.