Alternating voltage and current in a capacitive circuit
Initially the voltage is zero and the capacitor is uncharged. In the first moments the current increases quickly. Later, as voltage is increasing the capacitor is charging and the closer the voltage gets to its peak, the slower it changes, meaning less current has to flow. And when the voltage reaches the peak , the capacitor is fully charged and the current is momentarily zero. Following the peak the circuit reverses and the charge leaves the capacitor, so the current always arrives at a certain point in the cycle 90 degrees ahead of the voltage, because of the charging and discharging action.
To better understand why this is need to consider relationship between voltage and charge for a capacitor: CV = Q and relationship between current and the flow of charge I= dQ/dt = C dV/dt ...