Arbitrators have signed some accounts from disputes they were arbitrating, those are the initial signed accounts. Accounts that can sign other accounts are:
Accounts signed by arbitrators
Accounts signed by a peer that can sign where the signing happened more than 30 days ago
Signing happens after a trade in which one account is able to sign and the other is unsigned.
This means both buyers and sellers can get signed as long as the counterparty is a signer. Maker or taker, doesn’t matter.