The cause was easy enough to identify: Data parsed by Kuhls and her colleagues showed that drivers were speeding more, on highways and on surface streets, and plowing through intersections with an alarming frequency. Conversely, seatbelt use was down, resulting in thousands of injuries to unrestrained drivers and passengers. After a decade of steady decline, intoxicated-driving arrests had rebounded to near historic highs.
… The relationship between car size and injury rates is still being studied, but early research on the American appetite for horizon-blotting machinery points in precisely the direction you’d expect: The bigger the vehicle, the less visibility it affords, and the more destruction it can wreak.
Here in Portland it was because police stopped enforcing traffic offenses.
https://bikeportland.org/2023/08/08/portland-police-bureau-officer-admits-no-traffic-enforcement-messaging-was-politically-motivated-377939
Largely it was the Portland cops being petulant babies. Oh, we want oversight? Decrease your increase in funding?
Well, no more traffic enforcement, among other things. Speeding, expired tags, no plates, stolen cars, everything just went nuts.
Still hasn’t really recovered.