This is a rather trendy bun in Singapore without much history. It all started (pardon me if I am wrong) when the famous bakery chain (Bread Talk) started selling this. It is really a hit when it was first introduced in Singapore many years ago.
For the benefits of our local customers, pork floss is also termed as Rousong and as per Wikipedia:
Rousong, also called meat wool, meat floss, pork floss, flossy pork, pork sung, or yuk sung, is a dried meat product with a light and fluffy texture similar to coarse cotton, originating from Fujian, China.