It wouldn't happen simply by dropping something on the keyboard. You can change the positioning of squad members by moving their tabs/portraits (in the character screen) but the squad leader is unchangeable save under 2 circumstances (that I know of).
1st is the 4 Horseman encounter in unpatched versions. In this case, whenever you enter the encounter, the squad member with the lowest charisma becomes the squad leader. If this is the case, simply give your main character drugs that reduce charisma until he is at the lowest charisma of the group and re-enter the encounter and he will once again become squad leader.
2nd is if you are patched to 1.27 (possibly some earlier patches, but not retail version), and you extract the reaver elders in the tank instead of simply walking them into the extraction zone. In this case, the reaver elder you extracted first becomes the squad leader, and there is no way I have found to change the leader if this is the case. But in this case you can get a clone army so it sorta makes up for it.
Thats all I know. If you could've provided more info perhaps I coulda helped more but as you don't know/can't recall, thats all the help I can give.