In Morte d'Arthur, how does the series of events make the ending of the tale seem fated?

The ending seems fated because the characters go out of their way to prevent their tragic destinies. Earlier, Merlin prophesied that Arthur would be killed by Mordred, so the spirit of Gawain beseeches Arthur to make a truce with his son, even if that truce is only temporary. However, a series of ill-timed events force Arthur towards his fate, such as a knight drawing his sword to kill a snake, this action being misinterpreted as a declaration of war by the opposing side. Chance events like these make it appear as though a higher power were in control of the story's events.
However, it must be emphasized that Malory does not seem to argue that Arthur’s destiny was inevitable, no matter what he did. At the least, character psychology plays a big part in Camelot's fall. Arthur’s own weaknesses as a man culminate in the events which lead up to the fatal final battle: his extramarital affair with Morgause (which resulted in Mordred's birth) and his unwillingness to deal with the Lancelot-Guinevere affair before it is too late. Had Arthur exerted more self-control and wisdom, Camelot might not have fallen as it did.