Why is the bytestring 0xaa21a9ed used as the witness commitment header in SegWit? As per BIP 141, the witness Merkle root hash is stored in a scriptPubkey field of a coinbase transaction output with a header consisting of the 4-byte string 0xaa21a9ed. What is the reasoning behind choosing this string? 

Googling only yielded a reference to namecoin on a bitcoin-dev IRC log.
It's random.

The intent was choosing something long enough to avoid having someone accidentally create an output that matches, but short enough that it doesn't unnecessarily makes SegWit blocks larger.