If I understood it correctly, it was that the compiler suite shipped with newlib, which have or can have printf supporting re-entrant use (several threads can call printf in async manner), or not supporting that case (typically sync usage, blocking across I/O). The re-entrant ones use malloc/free for, iiuc, a per-thread buffer.

In many cases when you have smaller systems, you actually don't want your OS to do dynamic memory allocation, since it opens up for all kinds of bugs related to eg double-free, or use-after-free, or running out of heap, or too defragmented heap.

It's also easier to calculate memory usage (and thus be fairly certain it will be enough) if you allocate as much of the memory as possible as static memory. Hence why some coding standards such as MISRA-C disallows use of dynamic memory allocation.

Exactly what caused the lock-up here isn't delved deeper into, but it might be that there was not enough heap, or not a working malloc-implementation linked into the compiled binary (could be a stub), or could be that a non-re-entrant printf was linked in while they tried printf from several threads.