Since this is not a formal communication, there is no hard and fast rule on how it should be done. It probably falls into the same category as advertising copy: short, punchy, to-the-point, since the user's attention span will be less than a goldfish's.
A few other things going on here:
With or without the a, Ask Question is perfectly understandable in what will happen if you click on it. This falls under the less-is-more category.
The other possibilities: Questions and Unanswered are also available at this level of the menu hierarchy. Ask question is about the same size as Unanswered and the designer may have felt this was good. Only using Ask might be considered too short/small in comparison to other choices on that line. If Ask Question ws too short, one may expect the designer to do something to make it look the same as the others. The texts that are used are label_tags which acts as a clickable button, so keeping them the same size is good practice.