Is it right to not show up for something and not offer an explanation?

When having a formal celebration at a restaurant for my kids' college graduation, a couple who was very close to us (she is the godmother of one of my kids) never said one way or the other whether they were coming. The event happened, and they didn't show up or call ahead to formally send their regrets. I e-mailed them the next day asking what happened. They never really responded and hav been AWOL ever since. This doesn't seem right to me. Was I wrong to question them??