The rumors I've heard is that it is the "Oracle Golf Club Handshake Problem" in action: the current C-Suite of Wal-Mart got wined and dined and golf outing-ed a bit too much by Oracle who wanted a large contract expansion with massive revenue boost (for Oracle) and were promised a bunch of software resources/troubleshooting/consultant time in the next contract if they switched to far more Java. That sales job turned Java into a top-down mandate. It is a fun modern version of the "Nobody went wrong ordering more products and consulting from IBM" problem if IBM had an even more ruthless sales department and also owned too much of the database industry and too much of the Payroll/Time & Attendance industry (and worse, their legacy purchasing decisions and sunk costs) because no one has cracked down hard enough on vertical monopolies in the last century.
Allegedly, of course. I'm sure Wal-Mart PR will happily tell you it was their own idea and how Java is the language of the future again, or something to that effect.
Also, whether or not the rumors hold, it is hard to discount the pure Occam's Razor that it was just Wal-Mart's own idea to cut costs because of the simple dumb fact that labor costs in Wal-Mart's home labor markets for Java developers are certainly cheaper than .NET developers in the same job titles/experience levels. A mandate to switch to Java certainly was one cheeky way to do a slow layoff of expensive laborers without doing a real layoff.
While it is fashionable to hate Oracle, they aren't the only Java vendors in town.
As someone that does consulting across multiple programming languages, what I notice is that .NET is too late to the party in the server room wars.
For better or worse, UNIX and ideas derived from it, won the server rooms and cloud deployments, even Azure is mostly Linux nowadays (> 60% as per official numbers).
Due to its previous history, .NET has always had a bad name in UNIX shops (even with Mono), and most companies would rather use something else, unless they are a Microsoft shop, it could be Java, Go, or whatever.
As far as I am aware, Wal-Mart isn't a Microsoft shop and only acquired Jet for the business, not the technology, so like in all acquisitions something like that was bound to happen.
For many years the Wal-Mart purchase of Jet was lauded as a brilliant "acquihire" by Wal-Mart because they had lagged so far behind Amazon technically and Jet gave them the bootstraps to better compete. It has been said that the modern, competitive parts of Wal-Mart's tech platform is largely owed to former Jet engineers and their .NET (especially F#) forward development ideals.
I've never worked for Wal-Mart, I've only seen PR and blog posts and job postings. That is likely its own spin of what happened, too. But during those years Wal-Mart was seen as a stable "Microsoft shop" in terms of what they were hiring for in software developers (if you didn't mind relocating to Arkansas).
Externally, the Java thing did seem to be a "coup" that came out of the blue. Like I said, the easiest explanation is that Wal-Mart, a company famous for ruthless cost cutting and layoffs, felt like they had milked their acquihire of Jet for just long enough to stay competitive and decided the easiest "exit" for those expensive employees was to force a top-down mandated tech stack change to a known cheaper tech stack and gently encourage them to quit on their own initiative in the chaos. (In that Wal-Mart was already very quick to RTO, they had already played that particular card, many other companies are still palming to cause similar layoffs.)
The Oracle rumors are icing on the cake, sure. But also extremely plausible given that is how Oracle's business works today.
