After the birth of a foal, something began to go wrong; they began to quarrel more often, scandals, tantrums, every and in one moment, they realized that things cannot go on like this. So, they talked, everything discussed and they came to this decision: they need to divorce.
They realized that they can't a build a happy family like this; it's sad, but true. In the end, Big Mac became a single father, and Sugar Belle back to the Our Town (and then married to Party Favor ahahhahahaha).
Don't get Sugar wrong, she doesn't left her child; she visited him regularly