A simplistic explanation - not exact but will help.
When you put an AC signal down a wire some of it will be radiated out rather than going from end to end - similar to an antenna but very inefficient. And there will be quite a number of other circuits running VDSL2 in the same cable bunch as yours and everyone will radiate just a little - and it will all be "in band" - that is the same frequencies as your VDSL2 signals.
Your copper pair will absorb some of the radiation from each of the others and as it is not part of your signal, it will look just like noise to the modems.
In return for the other parties causing interference to you, your wires also radiate and cause problems for the other!
The radiation from one pair and absorption by another is called "crosstalk".
There is not much that can be done - twisted pairs helps but is not perfect, separation is also beneficial but impractical. Vectoring, which might come out with the 30Mhz scheme tries to predict noise based on various factors and then injects an inverse signal but is still not perfect. It is a technique successfully used in other areas including noise cancelling headphones ...
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