Coyotes don't mate for life. They may keep the same partner for a few years but it's not a lifetime thing. Kinda like humans - as long as your shit is tight, it's all good.
Actually the reason is that either the female gets too old to breed or the male loses dominance in the pack. Again, kinda sorta like humans.
Usually only one pair in the pack breeds and this is the dominant pair - I hate the term alpha but I'm going to use it - while beta dogs help rear the pups once they're born (63 days after mating give or take a day) by helping the alpha male bring home the bacon. Beta females will also babysit on occasion.
The only time another pair in the same pack will breed as well is if the pack has suffered heavy losses due to predation. I don't know this for sure but I've been told that dual litters also come when the rabbit/rodent population is on its' high cycle, about every 7 years.
Usually, only 2 pups out of the litter will survive their first year. They either die from disease, accidents, or older males kill them. Again, like humans.