Someone may have a more scientific explanation, but I think that, if we are dehydrated, then the body is not performing efficiently and this can cause all sorts of other mechanisms to kick in to try and rectify the problem, whether that is by making us thirsty, or retaining what liquid we can, or certain regulatory hormones being secreted etc. All this can have an effect on control because the system is 'out of whack'. I wouldn't be surprised if, in extreme cases, blood volume also plays a part in messing with our levels.
Hypo signs are usually caused by the stress hormones, so maybe these are not as efficient in a dehydrated body?