Several aspects to think about:
1.) Motor has 2R impedance, rated 1,7A @3,7V nom. for each phase, so supply of 8V is more than sufficient.
2.) AAA batteries cannot deliver the high current needed for supplying the motor. Voltage will collapse after short operation.
3.) A4988 has chopper operation with senseR to limit motor current. You are using a module, without knowing how it was dimensioned. Seems the module was designed to limit @2A, which overloads stepper (& even more stresses the batteries)