I read it was another pilot waiting to take off. But also this happened at Pearson. Depending on which runway, there are multiple places for people to park and watch takeoffs. Many do and film them.
The same reason any video happens to catch any incident at a specific time. Chance, luck, curiosity, no particular reason, etc. Everyone these days carries a camera and video is omnipresent.
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Don't turn this into some conspiracy theory.