Why voltage in capacitor lags behind the source voltage in RC circuit?
Imagine if the resistor is so large that the capacitor barely charges at all. Now consider what happens just after the source voltage reaches its positive peak. The capacitor voltage will stay below the source voltage for almost another 90 degrees. So the capacitor voltage will continue increasing through all that time.
It's easy to see it under these extreme conditions. Obviously, if the resistor or the capacitor is smaller the capacitor voltage will reach its peak sooner. But it will always be after the source voltage did since the source voltage will still be a bit higher than the capacitor's voltage and the capacitor voltage can't begin dropping until the source voltage is less than it is.