Over voltage.

I think an alternator can put out up to 50V, if the field is driven directly from the alternator output, and not connected to battery load. As the alternator voltage increases, so does the field, a runaway condition, only limited by magnetic saturation. Remnant magnetism in rotor starts the process.

It is true with battery connected the voltage may only reach 21V. The battery impedance is low, and an the impedance of the alternator limits the source voltage.

Making a guess based on lack of information... wiring problems.