Actually, the PIC maybe resetting itself due to internal reasons.
Have you ensured the watchdog timer is disabled? It is enabled by default.
What are you current Configuration Register settings?
If you are attempting to drive the stepper motor with the same power supply as the PIC circuit, it could certainly be the source of reset as previous reply suggests.
Post the schematic of your circuit so that we can advise you further.
Also the pattern output on PORTA does not appear to be standard pattern to drive a stepper motor.
What type of stepper motor are you driving?
A typical stepper motor drive pattern is as follows:
Unipolar Full Step Forward
1100
0110
0011
1001
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