Shares of airlines are broadly lower Tuesday, despite the day’s marking the start of the busiest travel period in what the Transportation Security Administration expects to be the “busiest ever” holiday travel season. The U.S. Global Jets ETF JETS was down 1.7% in afternoon trading, a day after closing at a six-week high. Within the Dow Jones Transportation Average DJT, the top six decliners are the index’s six airline components: JetBlue Airways Corp. JBLU shed 4.1%, American Airlines Group Inc. AAL slid 2%, Southwest Airlines Co. LUV gave up 2%, Alaska Air Group Inc. ALK lost 2%, Delta Air Lines Inc. DAL fell 1.9% and…