Why Did Isildur Cross the River?

[Inziladun]

Originally Posted by Ibrîniðilpathânezel

Given the pride that led him to keep the Ring against all counsel, he was vulnerable to the Ring's desire (if such it can be called) to return to its Master. Having Isildur take the less safe route along the river would work to its advantage, since it put him more directly in harm's way -- and on the quickest path for the Ring to be regained by the enemy. That's my thought, anyway.

According to the UT chapter, the Ring was a probably a factor in causing the Orcs to attack with greater courage and ferocity than they ordinarily would have; it called to Sauron's servants.
In LOTR we see it apparently acting in answer to other wills in concert with it, while they are in its vicinity. An example might be it slipping onto Frodo's finger at the Prancing Pony, in order to reveal itself.
However, would the Ring be capable of planning Isildur's route for him, knowing some of Sauron's forces would be waiting for the Dúnedain hundreds of miles away? The Orcs that ambushed him were part of a small, forgotten detachment, whose sole purpose seems to have been to waylay any enemies they outnumbered.

Originally Posted by Rumil

Although perhaps the Gladden Fields were more marshy and thus impassable on the Western side, making it a sensible idea to go along the Eastern bank of the Anduin. As long as no interference from Dol Guldur was expected.

Possibly, but they still could have made use of Khazad-dûm or the Redhorn Gate and avoided the Gladden Fields entirely.