Why is there a pull-up torque during the starting of an induction asynchronous motor
"Squirrel Cage" motor designers can change the characteristics of motors by modifying
  windings, rotor slot geometries, end ring size, rotor bar and end ring
  resistances, number of slots,number of rotor bars, amount and type of
  magnetic steel, etc.


ref http://industrialelectricalco.com/wp-content/uploads/2014/01/nema-abcde-torque-curves.pdf

By reducing the eddy current rotor losses from thick metal squirrel cages, the motor has less iron and less starting torque but more torque for fans at high speed when the slip speed is smaller.


Torque increases with speed squared as slip is reduced between rotational coupling of stator fields with moving rotor permeability coupling
Torque reduces towards zero at 0 slip speed due to back EMF opposing voltage applied to stator reduces available current to meet load demands as speed increases.
The mass of steel and geometry of rotor bars affect these variables where a peak can occur.