2010 Buick LaCrosse - Shifts on its own

2010 buick lacrosse cxl, while driving around 40mph, it shifts to the m1 spot. what is the cause of this?

For those of who don’t own one of these vehicles, please clarify “the m1 spot”.
What–exactly–does that mean?

The transmission is switching to the manual mode.


Any idea why it would do that on its own?