What does it mean when your touchscreen phone keeps on shutting down without you doing it?

Number of issues.

Have you set the phone to power save and if yes how long,

Is the auto shut down on in settings - chek teh time limit,

Is the battery failing - check full power charge to shut down time

Are there any apps running which is shutting down the phone, chck these as well.

In the marjority the issue describes power saving.