There was no understanding why but all of a sudden Mathew had breasts. Not pecs. Not man boobs he had full female breasts. And they weren't just for looks. They started to leak milk.
The problem was that Anna's stopped giving milk.
Kevin wouldn't take anything but breast milk from the source and it then became Mathews job to breast feed Kevin.
Right there in the mall Kevin demands feeding and Mathew sits down and holds him up to his breast like it was the most normal thing ever. But whoever made the change thought this wasn't enough for Mathew. His breasts increased to the size of sockerballs completely filled with milk.
Unfortunately that wasn't the only change that happened to Mathew that day and that night Mathew would experience the second part of his change.
He and Anna made love and Mathew became pregnant. His permanent milk producing jugs needed someone to drain them and whoever gave him the breasts made sure he used them.