I believe (but dont quote me on it) it is to do with someone on the flight having a baby, but not realising then can OLCI at T-72hrs, or even not being able to OLCI. Therefore they hold the bassinet seat until the last possible moment.
It happened to me on my MAN-LGW flight last week, i was unable to pick any of the first row and instead picked 2A. When i got onboard, i was behind 2 muppets who didnt try to keep a screaming baby quiet from doors closed until doors open.