The guy was certain that one of his allies had stabbed him in the back. A world cannot have more than one set of converts. This has an interesting little glitch. If a world is already fully converted, such as the Apostle's home world, then it does no more conversions. This is also an important point to consider if you have declared a Jihad against another Apostle.
Timing is a critical issue, though.
This happens even if they exceed the normal population limit of the world, and would have died off anyway. Instead, it runs around admiring the newly sterilized world, giving the berserker an extra five points a turn for a robotized world, starting immediately. This also works if you have two fleets, and one robotizes while the other bombs. That gives a nice marginal assist, but some players are greedy. They want it all, preferably in big hunks.