You witnessed the demise of the APU. Bleed air from the APU is used to start the engines, and the effort was too much for that one ! The APU is located in the aft fuselage.
The Big Blue Servisair truck is an Air Start unit. A mobile compressor that provides 40 psi air to start the engines in leiu of the APU, usually only when the APU is not available.
Two hoses are required to provide the volume of air needed to start the larger fan engines such as the CF6, RB211, Trent etc. The smaller engines on 737 series or similar only need a single hose.
That particular Servisair unit is the loudest one I have ever heard.