Too much current in the field circuit. Another cause, other than something shorted/ shorting, is TOO MUCH FIELD CURRENT. There is a procedure in the service manuals to measure that. It could be a partially shorted alternator field (rotor.)
If you have not, I would pull the alternator apart and give it a good inspection. Look for cracked/ loose/ other problems where the brushes might be grounding out, as well as checking field current.
What is with the wire coming out of the alternator Is that yours?