I have a first-contact scenario where I would like an ice age to be occurring rapidly. All the things that could change to make a fast ice age more credible: orbital period (403.42 local days), length of day (21.79 Terran hours), and surface gravity .67G. I haven't figured out how far that is from the star or what kind of star it is. My protagonist muses that the small molten iron core was part of the reason for the rapid freeze, as I already said the Van Allen belt is much weaker than Earth. I know this is ridiculously specific but can anyone point me to reference material I would like that - or pontificate, everything is welcome.
A runaway snowball earth effect causes your rapid ice age.
Regardless of the particular processes that triggered past glaciations, scientists generally agree that Snowball Earths arose from a “runaway” effect involving an ice-albedo feedback: As incoming sunlight is reduced, ice expands from the poles to the equator. As more ice covers the globe, the planet becomes more reflective, or higher in albedo, which further cools the surface for more ice to expand. Eventually, if the ice reaches a certain extent, this becomes a runaway process, resulting in a global glaciation.
Your world was mostly rainy before and there was not much axial tilt, so minimal seasons. There is a lot of land and less open water. Snow was rare and it did not stick. A volcanic winter event cooled down the poles enough that snow did stick and the albedo went up - more light was reflected and so the globe cooled. The more it cooled the more snow sticks. Oceans freeze and snow sticks there too. Higher and higher the albedo goes. Runaway snowball.