This has long been a feature of WoW, where it will automatically pick up a target if you attempt to use a combat ability and do not already have a valid target.

At the top of your combat macros, you can put something like
/stopmacro [dead] [noharm]

This should prevent the behavior you're seeing



Secondly if you have auto-assist enabled in ISBoxer, where you press 1 and it first assists and then attacks, you may end up targeting corpses (the previous target) because of the inherent delay in Auto-Assisting. http://isboxer.com/wiki/Assist_delay