HP ProCurve Switch all green lights when connected to another ProCurve

I have on one end of the network an HP Procurve 2510 with a SFP fiber transceiver connecting to a Cisco 2900 with a built in fiber port.  All works fine until I connect another HP Procurve switch replacing the Cisco and all HP Switches in both racks, a total of 4 of the, all green lights light up and stay lit until I disconnect the fiber.  After that every older switch in the network, mostly Cisco 1900 & 2900, need to be power cycled before they begin passing traffic even when setting things back to normal.  Does this mean there is a router loop, or will enabling spanning tree help, it is currently disabled.  It is a large campus network and cabling is questionable, but the fiber has been tested and is in good condition.