There are many reasons why a business might be considered high risk but here are a few:
You have too high of a chargeback ratio
You offer recurring billing, subscriptions, or a free-trial.
You sell high-ticket items.
You are in an industry that has a reputation of having high fraud and chargeback rates
Keep in mind that ALL merchant accounts carry risk. Banks simply want to make sure that their time spent on paperwork and disputes is worth the return.