MacBook Pro keeps booting into Windows

My MacBook Pro (which has Windows via BootCamp) now randomly boots to Windows on startup.

I will usually pick 'Restart' from the power options on the Windows login page, hold down the option key, and pick my Mac hard disk icon.

The problem is- now it will sometimes boot into Windows even after I have picked the Mac option. It's completely random when it will boot into Windows and when it won't.

I switch back and forth at least twice a day.