Why do metal objects in microwaves spark?

I heard that electrons accumulate at points on metals, and this clearly explains the arcing phenomenon, but how does a microwave make an electron imbalance on the fork?

Answer

Why do metal objects in microwaves spark

Those sparks are due to dielectric breakdown of air.

Microwaves are absorbed by metal and majority of it is reflected off with same phase, just like a mirror. However, because of this, there can be a large concentration of electric fields in some parts, it causes the massive potential difference that in turn lead to dielectric breakdown of air and the sparks you see.

I think this link explains it quite well and seems to be reliable: https://engineering.mit.edu/engage/ask-an-engineer/why-cant-we-put-metal-objects-in-a-microwave/

The oscillation of the microwaves can produce a concentrated electric field at corners or an edge of a metallic object, ionizing the surrounding air “so you can hear it popping away,” says Ross. You might also see sparking, which “is a little like lightning,” she adds. This kind of microwave sound and light show isn’t limited to metal. Ross sometimes puts on a demonstration for her kids: She cuts up hot dogs, creating sharp edges, and “watches the electric sparks jumping between them.”

As for the second part of your question:

I heard that electrons accumulate at points on metals, and this clearly explains the arcing phenomenon, but how does a microwave make an electron imbalance on the fork?

The electric field is strongest on sharp edges. Why? Well that has been asked and answered here: Why is electric field strong at sharp edges?

So yes, the concentrated oscillating electric field and resulting high electric field seems very plausible between the tines of the fork. That, in turn, causes the sparks.

Another related point worth noting is that the actual movement of electrons is considerably slower than build up of strong electric field. The electric field causes the dielectric breakdown.