Defensemen fall because they have a longer development curve. Forwards peak relatively early and it's clear what skills they're going to have at a young age. It's harder and a longer process to teach defensemen, and they mature later, so often the best ones in the draft aren't the ones that you would assume during the draft itself. That goes quadruple for goaltenders, who are completely unpredictable.
I'm fine with a general philosophy of drafting a forward in the first round and a defenseman in the second.