Self-absorbed people have a tendency to launch into conversation about themselves assuming you already know key points about themselves that they have never told you. This more often happens when they're seeing you again after some absence. It never occurs to them that you wouldn't know, because it's so important and obvious to them. This can make conversations with them confusing.
Sometimes, they get offended at you for not knowing things they haven't told you, because your not knowing them means you clearly didn't care enough to pay attention to things they didn't tell you. But this is a somewhat more extreme case.
(Thanks to cmeckhardt for helpful changes in wording.)