I don’t mind guessing. Based on the last comment about routing mark, there’s probably something like:
/ip firewall mangle
add chain=prerouting src-address=192.168.160.0/24 action=mark-routing new-routing-mark=xxx
where xxx is routing table that contains only single default route and nothing else, so it’s hitting 1) described here: http://forum.mikrotik.com/t/routing-mark-and-route-traffic-to-a-different-gw/160754/4
Problem is, without that comment, there’s nothing to work it. People need to understand that providing enough info is important.