RepRapFirmware is used with a wide variety of heating systems, including ones for high temperature plastics such as PEEK and PEI. So one of the main purposes of auto tuning in RepRapFirmware is to establish the characteristics of the heating system, so that during normal use the firmware can better spot when it is not behaving as expected. Until you auto tune the heater, RRF doesn't know what to expect from it. So it does minimal checks during auto tuning - in particular, it checks that the temperature rises shortly after turning the heater on.
The melting point of aluminium is 660C, so it looks like your heater cartridge was greatly over-powered. Were you by any chance using a 12V heater cartridge with a 24V supply?