carpet buckling

[facepalming]

we had carpets fitted on our stairs 2 months ago. they have been fine.

we now have the decorator doing stairs and landing and he has pointed out that the carpets buckling on a couple steps.

He said this is bad fitting of the carpet but to me it seems odd it's only occurred while he's here!

He had masking tape on the carpet but hadn't lifted it up yet.

He also said it should have been glued.

anyone know why this is occurring?