Interference is probably way down the list of potential causes. The first customer complaint was likely just one customer to begin with and could have reported

Interference is probably way down the list of potential causes. The first customer complaint was likely just one customer to begin with and could have reported that the issue was occurring at a different time of day depending on when they used the Internet. It wouldn't have been until they had multiple complaints and eventually a user who could pinpoint losing connection at 7:00am that they looked at it being something external to the individual customers connection.