These curiosities probably apply to all clubs, not just United.
The need to scapegoat: When we lose a single match many of our fans blame a player and once that player becomes a target it's difficult to shake off that reputation and they become the focal point of negative reactions. However some scapegoats can eventually turn it round then they develop a strange legendary status for having strength of character, players like Chris Montgomery, Chris Basham, Jack Robinson.
The underdog phenomena: Our support when playing small clubs where we are the favourites is pretty shit much of the time
but conversely when we play a big team where we are underdogs our vocal support and passion is really impressive.
Our history effects the mentality of the present fans: I've seen us lose so often (7 consecutive occasions) in play-off finals, decisive last match of the season relegation matches and cup semi finals that I now kind of automatically expect us to lose. You can feel it in our crowd, when ever we have a chance of success, our fans are asking each other "what do the footballing Gods have in store for us now". We've conceded so many late goals in these matches. We even had a penalty shoot out against Huddersfield where we were 2-0 up in the penalty shoot out. All the Blades I know were convinced we'd still lose that play-off final penalty shoot-out. Belief is a big factor in sports psychology, so we are like PSG in the Champions League, we know we'll mess up. We could be 3-0 up with 10 minutes to go and our fans still wouldn't feel confident. Different sets of players over the years but the same set of fans who seem to pass their psychological trauma/ negativity down to the players.
The recent social media phenomena to exaggerate with polarised views: Win 2 or 3 matches and we're brilliant, going to win the league. Then fail to win in 2 or 3 matches and we are crap, no chance of promotion. In fact we do seem to have loads of fans who thrive on negativity (is this a Sheffield thing?). We're a self deprecating set of fans. It doesn't take much until it's "sack the manager", "sack the board" and comments like "why would anyone ever choose to be a Blade as we'll never achieve a single thing, ever".
HOWEVER I don't think it helps that you have Blades pretending to be Owls and Owls pretending to be Blades.
So these people go into SUFC and SWFC social media groups deliberately trying to spread the negativity.