incase your adjacent wire's impedance is higher than your wire's impedance, then there will be a larger amount of electric field(analogous to voltage drop) across the higher impedance giving rise to cross-talks.
crosstalk is the phenomenon of the signal from a adjacent line getting coupled... the main idea here is that a small current across a high impedance causes a lot of voltage...
whenever a gate or something switches it consumes a lot of current and after switching this leads to the peaking up of current throughout the circuit (ringing current) and hence this causes significant change in the gates with high impedance...
this is why TTL gates which are not used are generally tied high or low(suitably)...