Why, in details, is the Velvet Underground called the gandfathers of punk?

The Velvet Underground influenced The Pistols, The Clash Iggy Pop etc. They are not what most people would consider "punk". But they were different than everybody else in the UK music scene at the time. They stood out away from the yardbirds and the stones and the beatles. I wouldnt say VU is the originator of Art rock (i would give that title to king crimson), plus you have to understand that music was just starting to branch out. VU was an influence. An influence that many punk rockers of that era had in common. Thats why they say they were the grandfathers of punk.