>The model then predicts a sudden shift in 2025 without any inherent explanation. There's a vaguely-defined downward-sloping "resources" line on the graph that seems to drive the sudden shift

What a fantastic question! You're thinking in the right direction.

The explanation involves systems theory, so I'll let an expert explain. https://youtu.be/HMmChiLZZHg?t=420 1h30m, but it's the best explanation of Limits to Growth out there.