Why are 0.13.1 Core nodes connecting only to Witness (e.g. 0.13.1) nodes?
Bitcoin Core 0.13.1 preferentially maintains at least 4 outgoing peers that advertise NODE_WITNESS support to avoid being partitioned. You are not only connecting to these nodes, more than 4 outgoing peers is coincidental or caused by those being other peers you recently connected to.


  even though, according to bitnodes leaderboard, these nodes form a minority.


Bitnodes only shows peers with listening sockets available. There is an unknown, greater number of nodes in existence which do not listen on a public port and will not be part of that leaderboard, but will connect to you. The preferential peering is exacerbating that effect, but is completely necessary.