Subdomain versus subdirectory

I think the issue may be due to "trust" - search engines usually treat subdomains differently from the main domain, so if you moved some pages to a different domain or subdomain then essentially you are "starting from scratch". Whereas if you use subdirectories the domain is already trusted and pages may be indexed more quickly.

If your traffic is increasing it could be due to any number of things such as more links being picked up, or just a single link from a popular site like digg or stumbleupon.

One more point of note: PageRank is page-based, not site-based, so if each of the pages had the same number of links in they will have the same PR.

i just came across this info graphic from slingshot that might be of interest :

_{(source: seomoz.org)}

I believe it's to do with page ranking. Search engines treat subdomains as completely different sites from the non-subdomain and thus by moving your pages from the subdomain to the main domain, they will be picking up it's PR (which may be higher?).

It may also just a coincidence. How long have you been studying the traffic for?