There's an optional flashback at the farmhouse the night after Year's Turn. In summary, it's about how Josiah convinced Thatcher to stay around after Abigail died.
Alma's shearing distracts him before he can have this flashback—or, if you prefer it when cause and effect happen in the right order, the kind of Thatcher that would burn down his own farm would never have flirted with Alma in the first place.