The viewer or director is sending the tapes
I don't know how to word this, but the entire movie is being filmed in the same way the tapes are being filmed.
Same format, same camera, even the same positions sometimes.
What I got from this movie is that it's not really a movie but real people under surveillance, hence the title "Caché", the cameras are verywhere, but they remain hidden to the characters.
To make an interesting 'film' the person or entity in charge of the cameras sends parts of their taped lives to different characters, this entity naturally knows everything about Georges' past as we see the cameras present during that time aswell.
We see a scene in which Georges is editing his show, he is cutting out parts which are boring and unnecessary, manipulating the footage. I think that is what happens i nthis film too, everyhting we see in it is footage from the hidden cameras, selected to make it fun to watch, tapes are sent to the right people to create drama.
I think that's also the reason why their son is aware of his mother's affair with Pierre, because he too has been sent a tape which contains proof of their relationship.
Edit: In the scene when his show is ending he receives a call and steps out of the studio, the camera which tapes the show follows him and now acts as the camera which shoots the film, signifying that he's oblivious to the cameras that surround him in his daily life.
Also in one scene Georges even references how surprised he was that he couldn't see the camera there right in front of his house when he walks past it on the tape and that it's position is awkward too, his wife claims it must be inside of a car but it doesn't look like it's been shot from behind a window.