I would guess they probably found that the "small business" segment of their customers ended up contributing to a disproportional percentage of fraud, support requests, etc, to the point they were net negative?

Though I still wonder if they considered just closing off to new customers instead of closing accounts of existing ones, since the fact that the latter would blow up in a nasty PR nightmare like this and do massive damage to their brand was pretty foreseeable.

Always a sign of questionable strategy when step one is fire your existing customers.

Charitably for Brex, if I am to guess the decision was not taken lightly, it possible the company cannot raise as much as it hoped, is running out of runway and has to cut costs and this is how its doing it - by downsizing, possibly layoffs in post-Aug

They've raised $1.5 billion in 12 rounds. I don't think this decision is due to a lack of funding; in fact, I think so much funding has significantly encouraged it. Unit economics and customer acquisition costs/lifetime total value become a lot more important when you're leveraged, doubly so with a recession looming.