When Mary Tudor (the elder) went off to marry Louis XII, she extracted a promise from her brother, Henry VIII, that when Louis died, she would be allowed to marry whomever she chose. When Louis did die a few months later, Henry dispatched Charles Brandon, duke of Suffolk to France to fetch back the former queen to England, with the caveat that Brandon would not marry Mary under any circumstances. François I of France was interested in Mary if his wife, Louis XII’s daughter, the sickly Claude, duchess of Brittany, were to die. Likewise, eager to prevent Henry VIII from marrying Mary off elsewhere, François encouraged Brandon towed the dowager queen. And the rest is history.
Now for the POD: the ship carrying Brandon to France sinks or Henry sends someone else to France to fetch Mary home (Norfolk or Buckingham as the only other dukes in England would be of sufficient rank? Buckingham seems unlikely (but that could be simply in retrospect given Buckingham’s later execution for treason)). Either way, Mary and Brandon miss the window to be wed (sure, they could always do it in England, but chances are that Henry keeps Mary under close watch to avoid something like this.
Mary most likely gets married off to her original betrothed, Charles of Burgundy (or whomever Henry wants an alliance with this week). What happens next? Especially if Henry still proves as incapable of siring a surviving male heir by Katherine of Aragon as OTL.