The year was 1526: The year of the Battle of Mohács, one of the most disastrous defeats in Hungarian history.
(And believe me, there are some strong contestants for that title)
It is not known exactly what happened. Historians claim that a nearby stream, the Csele, was flooding from the rains earlier, and while fleeing, the king fell in the stream and was dragged under by his armor. According to some sources either he or his horse was also wounded, and while trying to jump over the stream, they fell in a tangle. They found his body further down along the shore of the Danube, almost two months later.
The memory of this battle still lives in Hungarian minds as a humiliating defeat, and a turning point for the worse in the history of the Turkish wars (the Turks, on the other hand, regarded it as one of their most glorious victories at the time). The tragedy of the young king only made it worse, and the means of his death (not from battle wounds, but from drowning in a stream that under normal circumstances would not have been deep enough to drown in) only added insult to injury.
Here is another famous 19th century painting, this time by Székely Bertalan, depicting the moment survivors found the body of the king:
PLOT TWIST! An article was published last week about two doctors re-examining the sources about the discovery of the king's body, and they concluded that he might not have actually died from drowning - mostly based on how good a condition the body was in 7 weeks after death. They didn't offer an alternative explanation, though.