Besides the definition related to a nervous breakdown, "go over the edge" can also mean 'to go beyond the normal limit', or 'to go out of control'. 1) She is very upset and if anything else happens she could go over the edge. 2) When lifting weights, is important not to go over the edge into an overtrained state; you have to be careful. 3) Is the climate about to go over the edge? There is also the literal meaning: Records from the Grand Canyon show that a mule never purposely goes over the edge of the trail.

Imagine someone standing on the edge of a cliff. If they get angry or have a breakdown, it's like they've fallen over the edge of the cliff. Dis-as-terrrrrr!

It means that they have had a nervous breakdown.

it could reference that, mostly it means someone has made a mistake or gotten angry.. usually something negative in some way.