At first I wasn't sure why it contained a separate Unix line feed when you would already be able to detect a Unix to DOS conversion from the DOS line ending:

0D 0A 1A 0A -> 0D 0D 0A 1A 0D 0A

But of course this isn't to try and detect a Unix-to-DOS conversion, it's to detect a roundtrip DOS-to-Unix-to-DOS conversion:

0D 0A 1A 0A -> 0A 1A 0A -> 0D 0A 1A 0D 0A

Certainly a very well thought-out magic number.