This isn't a surprising move. Byrd is, to say the least, a long-serving senator from a coal state and he voted yesterday along with 25 of his colleagues to prevent the senate from passing climate change legislation through the reconciliation process. More significantly, he's the author of what is known (by sheer coincidence) as the "Byrd rule", which makes any provision in a reconciliation bill doesn't impact entitlement or tax law vulnerable to a point of order--and, therefore a 60-vote threshold. The Byrd rule is, like the filibuster, extra-constitutional, but it's also standard practice in the Senate, and, it's to be expected that its author takes a limited view of what belongs in a reconciliation bill.
The full text of the letter is available here.