You can use distribute lumped elements to analysis the transmission line. because transmission line can be equivalent as RLCG cascade ciruit, and between adjacent two lines exists mutual capacitances and mutual inductances, and then the coupling effects appears. another way is using the microwave theory, because the line transmit signals, there would be electromagnetic fields around the lines, when the frequency is low, the distance between two adjacent lines is quite little compared to the wavelength, then the coupling effects can be neglected, but as frequency goes higher, then the distance between adjacent lines is quite big compared to the wavelength, then the energy of the signal carrier line constrains around the line, so the coupling effects can't be neglected. i think it is the mechanism of the energy coupling between transmission lines. hope it can help you