I found a fare that has no reason or rhyme to it
I want to fly one way SEA YVR
UA's fare is over USD $200.00 plus the tax
If I book it as SEA SFO YVR the fare is USD $52.00 plus the tax
As has been said above, the fact that you don't understand the reason doesn't mean there isn't one.
Looking at one of the days that fare is available, UA is $82 total. Delta and Alaska both have direct flights for $73, and Air Canada is $84.
United knows that nobody in their right might is going to pay $200+ to fly via SFO. Some people will just pick the lower fare, but others might be happy to pay a little more to fly United for whatever reason, so they'll take your $82 to fill a seat that they likely decided they weren't going to be able to fill anyway and fly you via SFO.
United's options are "Charge $300 and don't sell any seats" or "Charge $82 and sell a few". If they thought their planes were going to be on full on that day they'd probably pick option 1 and save the seats to see to people flying SEA-SFO (for $200 as you said), but if they've predicted they'll have spare seats they might as well flog a few off for $82 and at least make something from them...
This is exactly why Hidden City Ticketing is a thing - people realize they can buy SEA-SFO-YVR for $82, but just fly to SFO. (And then get to the airport and get denied check-in because they don't have a visa/eTA/etc for Canada and blame the airline as they were only planning to fly to SFO anyway...)