S4: How did the enemy exploit sustainment weaknesses to contribute to the US withdrawal to the Pusan Perimeter?

The NKPA devised their plan to quickly and powerfully overwhelm any forces they came into contact with on their drive down into the South. They did this on purpose because they knew how the ROK was ill-prepared for a defensive engagement, and they knew that they would be able to force the withdrawal down further south.  So the NKPA executed their plan which worked perfectly. It led to mass confusion and piecemealed defenses, which then led to the orders to fall back south of the Han river and blow the bridges to try to prevent the NKPA from following. They ended up blowing up the bridges, but at the cost of an extremely hastily retreat, leaving massive amounts of supplies and equipment for the NKPA to exploit, which increased the value of their victory on the initial invasion.  The NKPA knew their enemy, they knew what their enemy possessed, and how their enemy would react, so they acted on it and achieved success.