Originally Posted by Sheldon Brown

That would be my guess too. A slack chan can wiggle so that the top of a tooth bumps into a side plate. That can generate a LOT of force on a fixed gear.
Not clear that it's a "monster load." Indeed, it doesn't sound like it to me. The magnitude of the load is related to the size of the sprocket driving the chain, not the ratio.

Thus, the pedaling load is _less_ with a large ring like a 48 than it would be with, say, a 42.

If the force was being applied by the rear spocket, as when resisting to slow down, a 17 tooth sprocket would not pull as hard a a smaller one, say a 15, would.

Sheldon "Loose Chains Break" Brown