Thief dual skills seem to prevent a number of utilities while they are progress. So while you are using unload, pistol whip, or flanking strike a utility such as a trap, shadow refuge, or basilisk venom gets queued to complete after the dual skill is finished. The queuing is lost if it dual skill continues too long (a second maybe). A heal will still interrupt the dual skill, as will another attack skill including scorpion wire.
I don’t know how important this is in the big scheme of things but I’m not sure why the dual skills are not responding properly to the players commands. Similar ranger utility skills such as traps will interrupt any attack skill and get used immediately.