If you choose one of the Preheat options from the control box (“Preheat PLA” or “Preheat ABS”), both the bed and the extruder start heating at the same time.
If you begin a print without pre-heating (whether from the SD card or from a PC or Raspberry Pi running host software), the heating process becomes a serial operation. The bed heats to temperature first, then the extruder begins heating.
There’s a very good reason for that behavior when printing. The bed, having a large surface area, takes a long time to heat up. The extruder heats up rather quickly. If you start both heating at the same time, the extruder will reach temperature first, then have a while to wait on the bed to heat. In the meantime, the filament in the nozzle melts from the heat and oozes out from the gravity. Additionally, this oozed filament leaves a void in the nozzle that has to be refilled before it actually begins extruding. The oozed string or glob of filament and the void in the nozzle could cause problems with the beginning of the print.
The extruder heating after the bed has reached temperature is by design to reduce the oozing before the print starts.
Use the Preheat options from the menu when leveling the bed, changing filament before a new print, or other such procedures. If printing from a cold state, just issue the print command and let the printer pre-heat in the order it is designed to so oozing is reduced.
Additionally, add retraction to the ending gcode to practically eliminate pre-printing oozing. Be sure to use a skirt at the beginning of your prints to prime the nozzle.