Well, I wasn't there, but I have been a corner marshal.
It takes a finite amount of time for each of these steps to happen:
- Corner worker at the station recognizes that there has been "an incident" nevermind oil on the track. Initially they're just likely to see someone go off track and/or crash.
- That corner worker might not necessarily be the one at the flag stand, so the message has to be relayed to the flag stand by hand signals.
- Then the flagger at that corner has to get out the flag and wave it.
- The radio operator at that corner has to call in to race control that there has been an incident.
- Race control has to call for the corner ahead of the incident to display a flag - who might not know that anything has happened, especially at Mosport, which is big and doesn't have good sight lines.
- All this while, the workers where the incident took place have to assess the situation and realize that there is debris/oil on track. It may take a finite amount of time. It MAY take until someone else crashes in it. Oil on track is not always visible the moment it happens.
- After the corner workers at the incident realize that there is oil on track, they have to display another flag - note that at this moment, the flagger has his hands full displaying both the waving yellow AND the debris flag. This is why the radio operator is a separate person from the flagger.
- The radio operator has to call in to race control.
- Race control then has to make a decision whether to have the corner ahead display another debris flag or to red-flag the session. Snap decisions to display a red flag are NOT made. It is NOT up to the corner workers - they can request it from race control but race control has to call for it so that ALL corners go red-flag at the same time. Corner workers have the authority to go yellow and/or debris based on an incident in THEIR corner but anything else has to come from race control - and it has to be that way, because otherwise the outcome will be chaotic.
All of this takes a certain amount of time. And yes, a number of bikes can get through before it all happens.