If the period of your course is longer than 10 months and it finishes between November and December, your visa automatically will expire on the 15th of March.
This means that you will have a longer period of holidays (yay!).
Another (possible) reason for many student visas expiring on the 15th of March is that between December and February is the summer season in Australia.
Meaning that is more appealing for tourism in Australia to have international students traveling around this period.
REMINDER
If you want to extend your visa, you cannot stay on holiday for longer than 8 weeks between the end of your current course and the beginning of another course.
So keep an eye out for the accurate date!
P.S.: To know the exact date of your course, check it on your CoE (Confirmation of Enrolment).