Part of the reason may be directly related to the unveiling of the Windows 8.1 beta to developers at Microsoft's BUILD conference this week.
According to the MetroStore Scanner website, which tracks the number of new and updated apps that appear each day in the Windows Store, submissions bottomed out in February at an average pace of 142 new apps daily. Since then, that average has steadily increased from 258 each day in March to 580 per day in June so far.
The total number of apps in the Windows Store has more than doubled since February, rising from around 44,000 to over than 97,000 today, a significant number though dwarfed by the number of apps in both Apple's App Store and Google Play.
(App store discoverability, generating app revenue and other developer issues will be among the key topics at the Tablet Ecosystem conference in San Francisco on September 12).