A hard fault occurs in Windows when a program accesses the Windows swap file instead of RAM.

Whenever the CPU needs to access a part of RAM that has been swapped out, the memory manager unit (MMU) throws a "Page Fault" instructing the Windows kernel to swap an old page out of memory on to the hard drive and swap the required page back in to RAM.

If you are out of RAM, the number of "Faults per second" will naturally rise.