From Wiki http://en.wikipedia.org/wiki/Mary_of_Burgundy:
Five years later, the 25-year-old Duchess died due to a fall from her horse on 27 March 1482 near Wijnendale Castle. She loved riding, and was falconing with Maximilian when her horse tripped, threw her, and then landed on top of her, breaking her back. She died several days later, having made a detailed will.
It seems awfully convenient that she suffered a tragic accient only a few years after the marriage and with a detailed will to boot.
There is no evidence to suggest that she was murdered, so no. I don't think it's suspicious that she died while horse-riding, nor that in the few days it took her to die she as a major heiress made a detailed will. It was fairly likely that she had detailed wills drawn up already (much like any major heiress today would have). There is no evidence of animosity between herself or Maximilian, nor any contemporary accusations of treachery or adultery on either side. In fact, this seems to have been a rather happy and loving marriage (as aristocratic marriages go).