Probably because you weren't the only person using the VPN to connect and either someone went miles OTT or too many people tried at once. Probably a mixture of both. It's worth noting that any blacklist will likely be across all Seetickets sites, not just the Glastonbury one so whatever triggered this may not have involved you at all.
So I'm 3 weeks behind conversations as per fking usual. So this Seetickets message is because of my work VPN - why has that only just started happening this year? Not that I need to buy tickets but still...