You asked why you would register multiple accounts. One reason could be to claim the welcome bonuses on all of those accounts, plus any other benefits for new players. That's abuse. Did you claim welcome bonuses or other promotions? @ndumm did he/she?

By using a VPN, you already broke their T&Cs. That isn't enough proof that the other accounts connected to the same VPN IP belong to you, but if they identified the same device being used for all betting accounts, it's definitely the same person using all of them. That is, of course, if what they are saying is true. I don't think they will share anything more with the public so as not to give other abusers any tips.