b) BTC/USD becomes rich on Bitfinex
c) people send BTC to Bitfinex
d) people sell BTC for USD on Bitfinex
e) people withdraw USD from Bitfinex
f) people send USD to other exchanges
g) people buy BTC, completing the arb
Note that selling BTC/USDT does *not* explain this. (a) you would likely just see USDT crypto outflows; (b) the BTC/USDT arb is generally kept in line between BFX and other exchanges.
I started at (b). What's (a)?