Why my iPhone 14 keeps changing the wallpaper every time I open it ?

Perhaps this could be why:

Rotate multiple photos as your Lock Screen wallpaper

1. In the Settings app, tap Wallpaper.
2. Tap Add New Wallpaper.
3. Select Photo Shuffle from the top row of options.
4. On the Photo Shuffle screen, set a frequency (On Tap, On Lock, Hourly, or Daily). Then choose a set of photos that will shuffle as you use your iPhone throughout the day.

1. 
2. Tap Done.