The higher the sidelobes of a signal frequency spectrum, the greater the "cross-talk" between transmission channels
Convolution theorem states convolution in time domain is the same as multiplication in Fourier domain, also the multiplication in time domain is a convolution in Fourier domain.

So when you truncate a signal (multiply it by a window), your signal's spectrum will be convoluted with the window's spectrum which widen your signal's spectrum depending on the main lobe width and also replicate signal spectrum depending on the side lobe height which is called frequency leak.
if you consider different channels over different bandwidth, when you truncate the signal the frequency leak will cause channel cross-talk.