If alcohol is a depressant, why does it makes certain people more hyperactive?
These types of reactions in pharmacology in general are called paradoxical effects.

The main effect of alcohol on the CNS is the potentiation of GABA receptors. GABA receptors are (mostly) inhibitory receptors in the CNS, so potentiation of GABA receptors mostly decreases brain activity by increasing inhibition.

However, you can distinguish between neural inhibition and behavioral inhibition. Some brain regions, like the prefrontal cortex, seem to primarily have inhibitory effects on behavior. That is, usually prefrontal cortex is active when suppressing actions. If you were to suppress the prefrontal cortex, you would get the opposite effect on behavior. When there is "inhibition of inhibition" the term often used is "disinhibition."

It isn't yet clear why exactly alcohol leads to behavioral disinhibition at lower doses, but this is one possible mechanism. In general, U-shaped dose-response curves are quite common in neuroscience, so alcohol isn't all that unusual in that sense.