Not too surprising - flights to and from Europe in the summer are often overbooked with leisure (read: no status) fliers, and when the economy cabin's full, they roll forward those with status first.

It does happen every once in a while, is a nice surprise, but should not be counted on. There are lots of threads here on being opped-up, pleading for an op-up, or ways to increase your chances of same.

It's happened to me a few times - it seems if don't expect it, it may happen, but when count on it, the chances are about zero.