This happens as a result of how EQ2 virtualizes input, IIRC. What happens is the game thinks your shift key is still pressed down. You can usually fix this by windowing/dewindowing, camping to yourself, or even sometimes playing with the shift keys until the game realizes you are no longer holding it down.
I do not really know of a way to prevent this from happening. In the rare case it happens for me (< once every couple months) I usually just window out and back and fiddle with the shift keys and that usually fixes it.
Of course as Drumstix said -- if you have sticky keys on that would certainly also do it.
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