My best guess, avoiding overflight fees in Canada.
looking at the actual route over past days, common theme seems to be maximizing the distance flown within US airspace (see 5th of Sep).
If i'm reading the below correctly TP is exempt from any overflight fees within
US airspace on that SFO flight
Fees in U.S.-Controlled Airspace
The FAA charges overflight fees to operators of aircraft that fly in U.S.-controlled airspace, but neither take off nor land in the United States.
As soon as they would cross over to Canadian airspace those fees pile-up.
For Canadian airspace there is a charge based on MTOW (basically per aircraft type) and distance See
appendix C-2 and C-3 for examples.
Back to the
actual route taken, on the 9th of Sep that jet stream must have paid off in less fuel burn vs the overflight fees.