(1) A lightning strike creates a transient voltage on the grid.
(2) Some power generators remove themselves from the grid to prevent damage to themselves from the transient voltage.
(3) Now there is too much demand on the grid -- too many people using power and not enough generators to supply it. (The line frequency dropping indicates that supply and demand are not balanced.)
(4) So the grid operator begins to (deliberately) remove some people from the grid to keep the supply and demand balanced.
What's not clear to me is what would happen if you didn't do step #4.
The Wikipedia article about the northeast US blackout of 2003 is a very interesting read: https://en.wikipedia.org/wiki/Northeast_blackout_of_2003