Yeah, stinks when that happens. Being flagged is the same as being suspended. The system has “flagged” your device for suspicious activity. As to what it picked up on, we’ll never know as Brave won’t tell you. If you know you did nothing wrong on the device that could be seen as an attempt at fraud or to abuse Rewards, then your best bet will be to create a Rewards Support Ticket at https://support.brave.com/hc/en-us/requests/new?ticket_form_id=360001302431 as SmartyAadi mentioned.
Just be advised once you create the ticket, it often takes up to 3-5 business days to get a response. They will contact you via the email address you provide in the support ticket. So keep an eye out on the email and make sure to occasionally check Spam/Junk folders just in case it ends up there.