MacBook Pro 2020 screen keeps magnifying itself inadvertently

I have a MacBook Pro 2020 retina running macOS Big Sur version 11.3.1 and the screen from time to time (more often than not) keeps magnifying itself automatically when in use.

This happens whilst just on the desktop and also when using no particular application. To rectify the problem I have to pinch the touchpad to restore to normal mode.

Can anybody suggest why this should be happening and how I may be able to solve the problem, please? Is it, just maybe, to do with the touchpad being inadvertently being touched when typing?

Thanking you

Sabina