What causes some aqueous solutions to have a low ph?

Low pH means the solution is acidic; if the solution is acidic, that means that there are hydronium ions in solution (H3O+). For example, hydrochloric acid dissolves into H+ ions and Cl- ions, and the H+ ions rreact with water like this:
H+ + H2O --> H3O+

If you want to get mathematical, pH is defined as the negative log of the concentration of hydronium ions.
Thus, if there are a lot of hydronium ions, the solution will have a low, or acidic, pH.  Hope this helps^-^