Tomek Korbak, one of the employees, who were fired, was the technical liason to METR for the investigation and states: "I was told verbally I was fired because of the way I communicated with METR".
Who made the decision to invite METR is not public knowledge, as far as I know. I imagine that an important decision like this was made on a much higher level in the organization.
Given the time limits placed on METR and the limits on what time frame they could investigate it seems plausible that they weren't meant to uncover as much as they did. The whole Hugging Face situation seems to have torpedoed OpenAI's hopes of IPOing this year so I'm sure there was a desire from investors, the board, or leadership for heads to roll.