As far as I can tell, extreme XMP bloat tends to go back to accumulation over time, e.g. when a document is reused again and again. I have no specific experience with Photoshop but could envision that when copying content from some other image that already has rich XMP data, such XMP data might travel with the copy&paste operation. Do this a couple of times, and 5000 all of the sudden doesn't look like a big number anymore
I once received a 50MB PDF file that had 20MB (sic!) XMP data in it... (all through an unfortunate chain of accumulation during numerous processing steps in the overall workflow).
At the same time I have the impression (but cannot prove it) that this was worst around Creative Suite 5 and 5.5, and is much better now (CS 6 and CC versions).