Maybe it was the gladiator costume?

The AP has an article, with picture, on Alagna's walk-off from La Scala's new Zeffirelli Aida.

More interestingly, Opera-L has a first-person account by one of the loggionisti (upstairs ultras whose booing triggered this incident).

I'm pretty much pro-booing but this idea of "traditional" booing as appropriate hazing isn't much to my liking.