Very likely this happened recently. AAAs happen spontaneously and aren't all that uncommon. They're also not immediately life threatening but are extremely serious if they rupture. It could even be mild and they're doing the surgery so he can possibly play again if all goes to plan. For most of us, a mild AAA may not be treated at all.
There's no way OKC knew about this. As far as I know AAA are predominantly picked up on ultrasound and this would be a standard component of a general physical. Letting him play with this would be absolutely risking his life. OKC weren't risking his life for a trade.
It's a [dang] shame but no conspiracy.