Are you sure it's not a (transparent) proxy trying to preload stuff ? And not being intelligent enough?
(do some packet dumps, look at the ttl (it's an indication of how many hops the packet that arrives has passed))
Is it a satellite ISP ? They do very "funny" things with protocols to get over their 1/3 sec additional roundtrip time due to hthe signal having to go 4 time the distance to the geosync orbit.