Why is water coming out of my airbrick?

without looking at the issue it is impossible to say what is happening, although you say the pointing ect is ok , this is a solid brick building built with lime mortar, if it has been repointed with a cement pointing this could be holding in water behind the cement pointing and this is what you see escaping, the air grill you are talking about is most likely for venting your floor providing it is still a suspended wooden floor their should be more than 1 grill and these should be at least 150mm above ground level. if you are concerned get a local specialist in to take a look and see what's happening, pick one that specialises in older property's and how the materials used to build then keep them dry. It basically impossible to say without looking what is causing your issue or to tell if its one thing or an other lt generally means looking at every conceivable entry point to establish the correct cause of what is creating the issue,their are so many different things that can allow water ingress into the house brickwork,roof,ground level,guttering so Iam afraid that is a question that cannot be answer blind good luck Alex