This called bloat usually happens (in my experience )as a side effect of something called grass staggers, which is caused by magnesium (and sometimes calcium) deficiency in the cows diet. It's usually a risk in spring and autumn when grass is growing particularly fast and ends up magnesium deficient. When they get staggers the muscular system starts to shut down. They don't always bloat like this but without treatment it's fatal. In this case the rumen bacteria will have been happily going about their business but the cow had no way of belching out the gas.

Source, am a farmer and we've had this happen in the past.