Just a thought - if I get up and have no bolus - even when I'm not eating yet - I will always, always go high. Perhaps it's as simple as 'foot on floor syndrome'?
For instance, if I'm getting up and going to the gym or for a run, I always have breakfast when I get back, but I still have a small bolus of insulin when I get up even though I'm not eating.
So it might be that the delay in breakfast (as you are using some time to change your cannula) and therefore the delay in breakfast insulin on the mornings you are changing is making a difference. Just a thought!