In part. It was money borrowed by the state. That means when it can't be paid back, it's automatically a systemic issue. And it was money borrowed to fund consumption. There was no good reason to ever expect it to be paid back because it wasn't funding productive activity.

> if this turns out to be a bubble it will be taxpayers who end up fronting the bill

Very possibly, particularly if part of the package are e.g. federally-subsidised loans. Before that, however, private parties will almost certainly lose tens if not hundreds of billions of dollars. That cushion, together with those parties being spread between domestic and foreign sources, is what makes this less risky to the United States than similar relative-magnitude projects in South America. (Plus the fact that this is a capital asset versus consumption.)