Coming from an Engadget interview with Sony's Shuhei Yoshida...

"The first time was because I had my Twitter account in my profile and that's against the rules. The second time is because I wrote, 'I love PS.' You're not supposed to promote a commercial product in MiiVerse, so they correctly interpreted 'PS' as 'PlayStation.'"

Sounds like Yoshida took the bannings all in stride. Let's hope he manages to avoid a third ban!