Don't know what air pressure you're running. +1 to letting some air out if you're near the max. The additional contact patch should make a difference. At those speeds my 35mm supremes feel quite stable. As mentioned debris or wet painted road makings would also make a difference.

One other possibility is a type of pavement (can't recall the proper term for it) that is no longer used for roadways. It is absolutely treacherous when wet. An on ramp to a nearby interstate was repaved a couple of years ago because of the number of accidents when the ramp was wet.