Reading the comments above, I can think your volume up button may be broken? But remember, you can perform a factory reset from the Fastboot mode too, simply connect your phone to the computer, install Minimal ADB & Fastboot (Windows), run it then type
fastboot -w[press enter] the phone should be reset.
Also ADB lets you do many things on your phone including factory reset:
Enable ADB (USB Debugging) from the Developer's menu
Connect the phone to the computer
Run Minimal ADB & Fastboot
adb shell [press enter]
wipe data [press enter]
ADB can let you boot in recovery mode and perform factory reset manually:
adb reboot recovery [press enter]
PS: Don't forget to install drivers to let your phone be detected by the computer.
Not on Windows ? Check this question related to a minimal installation of ADB for all operating systems.
Check also our ADB tag-wiki