It's fairly simple, really. Animal rights activists want to prevent anyone from having any contact with animals. So, as part of their attempted emotional manipulation of people to get them to "accept" that any human contact with animals is unspeakably cruel and must be stopped, they have to demonstrate to people the horrible abuse. Since that level of abuse is extremely rare, especially in professions where animal health is crucial to success, they have to infiltrate those businesses, wait until everyone else is gone, and then abuse the animals (or pay someone to abuse them) while they film. They then have all kinds of "evidence" of animal abuse occurring in whatever industry they target, whether it's farms or laboratories.
It's not much of a conspiracy, really. It's not like getting a job on a farm, at a slaughterhouse, or in a research institution is incredibly hard. It's not. In labs, at least, it isn't unusual for people to be working outside of normal business hours, so the opportunities are there for the activists to be the only ones present.
Mercy for Animals has a vested interest in making this sort of a video . . . it does not necessarily follow that the video is fabricated, or that the actors on the video share that vested interest.