Why is the generator turned back off for starting the second engine on a King Air? The engine start procedure for the Beechcraft King Air B200 says (emphasis mine):


  
  Right Ignition and Engine Start Switch – ON [R FUEL PRESS] annunciator – OFF
  Right Condition Lever – LOW IDLE (after N1 rpm stabilizes; 12% minimum)
  ITT and N1 – MONITOR (1000 oC maximum)
  Right Oil Pressure – CHECK
  Right Condition Lever – HIGH IDLE
  Right Ignition and Engine Start Switch – OFF (at 50% N1 or above)
  Right Generator – RESET, then ON. CHARGE BATTERY until loadmeter reads approximately .50, then OFF
  Left Ignition and Engine Start Switch – ON [L FUEL PRESS] annunciator – OFF
  As Left N1 RPM accelerates through 12%:
  
  
  Left Condition Lever – LOW IDLE
  Right Generator – RESET, then ON
  
  ITT and N1 – MONITOR (1000 oC maximum)
  Left Oil Pressure – CHECK
  Left Ignition and Engine Start Switch – OFF (at 50% N1 or above)
  Left Generator – RESET, then ON
  Right N1 – REDUCE TO LOW IDLE
  


Why is the right engine's generator turned back off before starting the second engine (step 7) and then only turned on again after injecting fuel (step 9)? Doesn't this put unnecessary load on the battery?
Yes it loads up the battery, but the battery is sized to do this. The generator is not necessarily sized to do this, especially on the ground, so this procedure gets you to take the generator out of the system for each start.

On an Alternator, like on your car, there is a phenomenon known as 'forward-stripping', which is where there is such a demand for current out of the generator that it overloads the rectifiers (that convert the created AC electricity into DC for output). So here, during/after the engine start, there may be a large charge request by the battery which might forward-strip the gen's rectifiers.