I think this is very common practise now, I know I encountered it last year:
Dropped my ND off for a service and MOT having requested new tyres to be fitted as they were no longer legal (just)
They did the MOT first, failing it on one tyre before carrying out the tyre change as requested and passing it.
What @BRUCE225 said makes sense though as they also noted my brake pads were low so I had them replaced.
This would have no doubt doubled the labour costs if they had done tyres > MOT > anything else as the wheels would have been off twice etc.
I know @bally3 had a similar situation with one of his previous MOTs: