What major event actually encouraged the growth and development of grocery stores such as the original Piggly Wiggly and why?

The introduction of "self service" ?Piggly Wiggly

Well, that is a new concept rather than a "major event".

The growth of suburbia?

that is certainly an important factor but this is "an Event" .. a specific time identifiable event that directly affected the growth and promotion of grocery stores. It was a more or less global event.

I probably gave it away, but the second part of the question "Why" would still need to be answered.

Was it the return of troops back home from WWII?

Well, you are correct in a way. WWII was the reason Grocery Chain stores really took off. Many of the smaller mom & pop food stores ended up closing because the men of the business went to war.