I think another reason for the increase in signups is Google's latest marketing campaigns. They have been starting to aggressively advertise the Chrome App Stor

I think another reason for the increase in signups is Google's latest marketing campaigns. They have been starting to aggressively advertise the Chrome App Store much more than before. Just yesterday I saw an entire commercial with the Angry Birds developers discussing how great apps on Chrome are. Google knows its Chrome store is legit now, so they're ready to show it off to the world in an attempt to make it as mainstream as the iTunes store is now.