Serperior rose to OU when it released with Contrary, which is its Hidden Ability. We'd known since 2010 that Serperior has this ability, but it was wholly unobtainable until it was finally released in an event in early 2015. Contrary is among the best abilities in the game, which explains Serperior's dramatic rise.
SubSeed isn't Serperior's only good set in OU, and truthfully isn't its best set either. Running Leech Seed means you give up either Hidden Power or Glare, which are important moves that increase the range of Pokemon that Serperior can do something useful against.
Serperior usually runs either a general Substitute set with Hidden Power and Glare, or a Choice Scarf set that swaps Substitute for Knock Off or Defog. The Substitute set is good against balance teams, which have plenty of passive utility Pokemon that grant set-up opportunities. The Scarf set is better against offence, since it outspeeds staples like Tapu Koko and Greninja, and has less offensive counterplay.
I've also seen Serperior used as a Heatran lure with Defog, Hidden Power Ground and Synthesis, which is good for offence teams that need a response to that Pokemon, as well as hazard control. Serperior is pretty versatile despite its rubbish attacking movepool.
Couldn't tell you why Serperior in particular gets Contrary, though I doubt it's supposed to relate directly to its 'regal' species title.
