It happens because the completion rate is calculated in a 60-day window, so in oder to keep it up you have to complete more orders. Let me explain with an example:
On day 0, you had 20 completed orders, that is 100% completion rate.
On day 1, when you got your cancellation, you had 20 orders completed in the last 60 days and 1 cancelled, so that is a 95% completion rate.
On day 2, a day has passed, and 3 of your completed orders are now too old and outside the 60 day window, so you have now 17 completed orders, and 1 cancelled, that is a 94% completion rate.
On day 3, another day has passed, and 2 of your completed orders are now too old and outside the 60 day window, so you have now 15 completed orders and 1 cancelled, that is a 93% completion rate.
And so on. Just do a simple math calculation: completion rate=100-((cancelled orders*100)/(orders in 60 day time window))
In order to avoid being demoted, you will have to complete enough orders to keep the percentage over 90%.