Discover Credit Card Closed Due To “Unusual Pattern of Risk” on Account

I had gotten a Discover Credit Card in late June and I was using it primarily for gas purchases. On July 22, they decided to close my account due to "Unusual Pattern of Risk" on the account.

Could it had been due to me using the card for just gas purchases, that they considered the account risky? Also had something like this happen to anyone else?

Also will this affect my credit score significantly? As they also said that they would report this to the credit bureaus. I do have a Paypal Cashback credit card that I use for my other purchases with a $5000 limit which I had for 6 months. I am a authorized user on a Nordstom Store Credit card that has a limit of $4000 with a age of 4 years. The Discover card had a limit of $1500.

Any information/insights would be appreciated.