The way I read it was the early spring thaw was to blame.
> The extra moisture and early spring thaw turned the Hungarian plains into marshes and swampland.
As in, they expected frozen ground not marshland that time of year. But reading the abstract of the original source, it seems like it was a combo.
> Documentary sources and tree-ring chronologies reveal warm and dry summers from 1238–1241, followed by cold and wet conditions in early-1242. Marshy terrain across the Hungarian plain most likely reduced pastureland and decreased mobility, as well as the military effectiveness of the Mongol cavalry
> The extra moisture and early spring thaw turned the Hungarian plains into marshes and swampland.
As in, they expected frozen ground not marshland that time of year. But reading the abstract of the original source, it seems like it was a combo.
> Documentary sources and tree-ring chronologies reveal warm and dry summers from 1238–1241, followed by cold and wet conditions in early-1242. Marshy terrain across the Hungarian plain most likely reduced pastureland and decreased mobility, as well as the military effectiveness of the Mongol cavalry