i believe that can happen if a transaction between the server and a linked server gets rolled back, but never completes the rollback.
if you check sp_who2 for spid = -2, which is indicative of the issue, i think that's the problem;
only way to fix it is to bounce the server, which rollsback the transaction without waiting for the Distributed Transaction Coordinator to report successfully rolled back @ the linked server.
Lowell