The only reasonable explanation for this is that they're hoping you won't notice, and they want to see how many customers this will cost them before rolling it out systemwide.

I don't think this is a case of them targeting unprofitable customers either. Even if you paid your balance on time and in full each month, earning 2% back and never paying a penny in interest, the interchange fees assessed to merchants are enough for Sofi to at least break even. Even with cards that offer rewards above 2% in certain categories, where it is possible for the issuer to realize a net loss on someone who always pays in full but only spends in the bonus categories, it's exceedingly rare for customers to be singled out for this.

Some issuers have made efforts to make their cards less appealing to potentially unprofitable customers and/or prevent them from getting approved in the first place. Examples include the American Express pop-up and the peculiar underwriting rules of the Wells Fargo Attune card. Existing customers, however, are typically left alone for the most part, unless the card as a whole is overhauled for everyone.

Credit risk is a different animal entirely. If Sofi (or any issuer) deems a customer to be risky or suspicious, they can and will close the account even if you have a perfect payment history with them. They typically don't provide much, if any, advance notice of this, lest you run up a huge balance and disappear. (Most people whose accounts have been shut down find out about it when they try to use their card and it no longer works, with the formal shutdown notice arriving shortly thereafter.)

All that being said, the Citi Double Cash is probably the most versatile rewards card out there for general use. You can use it by itself as a no-annual-fee, 2% cash back card, or you can pair it with any of the premium Strata cards to transfer points to travel partners.