I think the longer a series goes on, the more likely the authors/creators will have their characters do things that feel "wrong" for that character. Mostly what I think it is, is that the authors are rushed and we don't get to see exactly how the characters got there.

My prime examples are Remus Lupin from HP early in book 7 where Harry fusses at him and pretty much Willow the entirety of season 6 of Buffy.