It's not intended to be, but perhaps it has to do with the imp chat.. for me when I use to many skills to fast, the imp chat marks it as spamming and disallows me to continue using skills for a few seconds. This includes, perhaps, pressing silence 3 or 4 times, then ablade 3 or 4 and so on and so forth in rapid concession. Maybe because backstab uses the line "so and so have been backstab" or something similar to that; that each time I backstab some of them are being cancelled out. Considering, hitting someone from the back automatically produces the /skill backstab that normal button binds/ items binds do. Just some thoughts I suppose..