Whatever it was - he failed in one of the most important duties of the emperor: to make sure that after your death there wouldn't be a devastating civil war.
In cases like that - if you don't make a decision about your successor - you jeopardy thousands (maybe hundreds of thousands) lives of your people.
But this sort of thing happened a lot IOTL in the ERE, so I don't think Alexios' behaviour is unusual by any means. As far as he's concerned, the succession is up to God, not him, and so his own deciding on a successor is largely irrelevant to the matter in hand. This sort of thing can be sort of hard for us to understand in the 21st century- Alexios would have been extremely strange in a Byzantine context not to believe in frequent divine intervention in this sort of thing. For Alexios to seriously question the fact that the succession would be chosen by God is somewhere akin to a modern leader seriously questioning, say, gravity.