Next time before dropping such connection, you can run
Select Userid , IQconnID , NodeAddr , ConnCreateTime from sp_iqconnection() where ConnHandle = NNN
NNN is connetion handle number which you are dropping.
This will give you some idea about who ( Userid ) opened which IQ connection ID ( IQConnID ) from which client box ( NodeAddr ) since what time ( ConnCreateTime )
Since the connection is already dropped, this information can also be found using IQ diagnostic files ( .iqmsg , .srvlog ) where you can look it up what exectly got disconnected.
Based on what was disconnected you will be able to figure out whether there is another user OR some other application opened the connection which you dropped.
Regards