1951 Admission Boom to the NPC

[violetpretty]

Paging all GC NPC history buffs! I had a question about GLOs joining the NPC, and in doing a search, I learned more than 26 groups had been members at some point, but it didn't answer my original question.

The following is a summary from the NPC Trivia thread on page 48 by NutBrnHair:

Basically, there were 7 groups that founded the NPC in 1902. Then, a couple of groups joined each year, with Kappa Delta being last of the first 18 groups to join in 1912. In 1933, three groups joined, Alpha Delta Theta (later absorbed by Phi Mu), Beta Phi Alpha, and Phi Omega Pi (both later absorbed by Delta Zeta ).

Then, there was no action until 1951, when 13 groups joined. They included the 8 current members that joined in 1951, plus Theta Upsilon, Delta Sigma Epsilon, (both later absorbed by Delta Zeta), Theta Sigma Upsilon (later absorbed by Alpha Gamma Delta?), Beta Sigma Omicron (later absorbed by Zeta Tau Alpha), and Pi Kappa Sigma (later absorbed by Sigma Kappa). Iota Alpha Pi was added as an associate member in 1953.

So, my question is, why was there this sudden boom in new NPC groups after many years of stagnancy? Did policies change? If so, what were the policies? I find it interesting, since there are basically two categories that the "1951 members" can be placed into: educational sororities, and sororities founded to cater to religious groups other than Protestants (or non-religious). I read somewhere in another thread in my search that the AES was admitted as a group to the NPC, but that still doesn't explain why all of the "religious" ones were admitted at the same time? Is there a reason or is it pure coincidence?