For the microstrip antenna, its impedance behaves like a shunt RLC tank at the first resonance freq, while it resembles a series RLC circuit at the second resonance freq. As a rough estimation, the second resonance freq is about twice of the first one. With respect to your case, where there are two troughs around 4.8 GHz (=2*2.4), the general rule of the second resonance freq still holds. The two-trough phenomenon is mostly due to the subtle variation of the impedance value around 4.8 GHz.