It is rather stressful for the leaving coworker to answer these questions. Just put yourself in their shoes. He/she could be thinking "I don't wanna see these guys anymore", and you're asking at the same time about the reasons. To begin with, try to ask about their plans for the next several months, their future work etc. If they're super excited about it, maybe they just need another challenge. That's totally OK. Wish them the very best and be thankful for the work done together. If they say "oh I dunno, their MacBooks are slimmer", then probably the main reason lies within your company.

If the resignation caught you by surprise, it's unlikely he/she will tell you the real motivation. People often start the contemplation long before they decide to leave. IMO.