Assumptions based on contributing factors. You had people more or less hold off on everything for a year, which tanked suppliers. Now that there's light at the end of the tunnel, everyone is buying making up for lost time, while supply hasn't recovered and realistically aren't going to double their capacity for what is seen as a short term spike.
With lumber prices where they're at, you'd see mills popping up like oil wells if they had any hope those were permanent increases.