Why tilt is formed for square wave input in highpass filter?

Lets have an example of simple RC high pass filter. Here, we take output across Resistor(in HPF). The tilt is because of charging of capacitor. you can say, as capacitor charges (ofcourse with voltage) the same amount of voltage has to drop across resistor ( to follow KVL). Since we are taking output across Resistor, so we see small voltage drop (as tilt). This can be minimized by keeping RC time constant large.