The cynical part of me is wondering if they're all jumping on the opportunity to do some "reorganising" with ZIRP as a get out of jail free card.
15% which is 100 people in their case isn't even that much, they could have implemented a hiring stop and trimmed low performers over the next 6-12 months ending up with the same headcount.
Wild guess 2: Since they are letting go so few people, they'll do another round in the future.