A good example of the start of scion (Viognier in this case) rooting because the under-vine weeder has mounded up soil around the vine trunk.
If this continues, you end up with the situation in the second photo: large roots growing from the scion but the rootstock fails to thrive. Eventually the rootstock dies off and the vine is own-rooted (and therefore susceptible to phylloxera and other soil conditions they may not be well adapted to...).

