I point you to Sefer Tamei Hamihagim siman 596 and 597.
The key is a response to various pesukim regarding treasure rooms and parnassah.
This particular Shabbos is chosen because it is the first shabbos that we can eat all grains (when it all turns to yoshon); It is also the first Shabbos that the Jewish people totally relied on grown grain upon entering E”Y as the Mon (manna) that was saved from the desert ran out.