If the investigation into Monday’s explosion at a downtown Fort Worth hotel finds it was caused by a natural gas leak, the incident would be part of a worsening trend of such incidents across the country, according to energy sector watchdog groups. The massive explosion at Sandman Signature Hotel, which left 21 people injured, has not been tied to an official cause. But Fort Worth Fire Department authorities have said it was likely a gas explosion, and representatives of Dallas-based natural gas distributor Atmos Energy have been on the scene. Since 2010, incidents like fires and explosions ca…