A lot of viruses insert themselves into your DNA, they may mess up the 3D structure, or during DNA repair result in misrepair / duplications, or simply insert somewhere and break something important. All of these are ways that can contribute to kickstarting or accelerating cancerous growth.
Hmm. Compared to what measurement? Most viruses are actually not oncogenic.
From cancer causes, oncogenic viruses are thought to be responsible for about 12% of human cancers worldwide:
https://www.mdpi.com/2079-7737/14/7/797
From what I remember, most viruses are not oncogenic in nature, so I am unsure whether the statement made is correct.