So, I’ve been thinking about the Mystery Trio lately and what it might have been that caused McG to leave.
We know that his laptop was found in the bunker where the Shape Shifter was kept, and that the Shape Shifter was found as a mysterious egg.
What if the first time the portal was opened an egg came out, and after it was discoverd that the Shape Shifter was too dangerous they decided to put it into cryostasis until they got the portal working again. As we can see in the episode “Into The Bunker” one of the cryo-tubes is broken open. Maybe that was the thing that went wrong.
McG didn’t want to help with the portal anymore because he was afraid of what other horrifying and dangerous creatures could come out of it! Sadly, without McG and his expertise in engineering, Stanley and Stanford may have made a few miscalculations and the portal became too powerful, eventually leading to the events of July 4th 1982.
The date that was probably chosen so that the lights and sounds coming from the portal could be mistaken for the fireworks of Independence Day.