Okay here's my assumption. People who go on about how their horse is an OTTB. As in mention it every five minutes. OVER COMPENSATING. Like short man syndrome. Probably don't ride great, need to make themselves look better.

People who have a TB and call it an OTTB despite it never having raced or worse never even having done race training is just laughable. Overcompensation to the extreme. As a side effect, pretty unlikely to take anything they say seriously.

Erm, those are the main two I think. Everything else people have already said!
Those same people are the ones who constantly remind you of their OTTB lineage. He is related to Secretariat, Seattle Slew, Man O' War! etc. I guess they don't realize nearly every race horse ever is related to those horses