How many hairy crazy ants does it take to short out an electrical system?
If the conductors are 1 ants length apart, then one ant is all it takes.  If they are 10 ants lengths apart, then 10 ants, if they go top-to-tail.

In reality it will take more as they move around lots.  Also, it is probable that it would be a gradual build-up of dead ants.  As an ant gets electrocuted it will curl up and/or explode.  After a while, the bits of dead ant will eventually bridge the circuit.

As for why... well, who knows what goes through the mind of an ant? (besides 110V)

It is well known and documented that mice chew through cables because of the 50/60Hz buzz they produce.  Maybe the ants are attracted by the EMF exciting certain areas of their tiny minds?