The pattern, called semelparity, is synchronised: mating in a given population is confined to roughly two weeks, timed by day length so that weaning coincides with peak insect abundance. During that window males abandon feeding, and individual copulations can run for hours.
The physiology is well characterised. Circulating cortisol rises steeply while the proteins that normally bind it fall, so free cortisol reaches sustained levels that few mammals tolerate. The result is gastrointestinal ulceration, breakdown of muscle protein and collapse of immune function, leaving males vulnerable to parasites and infection. Within a few weeks the male population is gone, and females raise the litters alone. Some females survive to breed a second year.
The standard shorthand, that the males die of exhaustion from sex, gets the mechanism wrong. Death follows from hormonally driven immune and tissue failure, and the leading explanation for why selection favours it is that concentrating all reproductive effort into one competitive burst outperforms surviving to a second season under these conditions.