No idea except for, doing it when he can come in within a few laps and take it with his normal pitstop is very... odd. Because it should both have been a 15 second penalty and done in the first 5 laps. even the 5 second penalty in the first 5 laps would have dropped him to the back, waiting 15 laps means he loses 5 seconds not 20-25, and because of the stretching out of the pack he loses only a few extra places.
The cynic in me says they waited purely to make that as easy as possible for him.
If they gave him the 5 second penalty on lap 1 he could still serve it on his first pitstop, learn the rules before getting all paranoid.