The quick high voltage casts seem to coincide with a new ability being gained. The 25% increments seem to reset the boss ability timers, thus leading to a quick recast. This is even more speculative, but the abilities themselves sometimes take a while to execute (cannons, laser), and may compress the normal boss melee attack timer to cause the double attacks.
It may prove useful to have your backup interrupt routinely handle the high voltages immediately after 75/50/25 ability gains. It might not be necessary most of the time, but the one or two saves will make your healers happy.
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