What are the origins of this nickname?

In early newspaper accounts of Alabama football, the team was simply listed as the "varsity" or the "Crimson White" after the school colors.
The first nickname to become popular and used by headline writers was the "Thin Red Line." The nickname was used until 1906.
The name "Crimson Tide" is supposed to have first been used by Hugh Roberts, former sports editor of the Birmingham Age-Herald. He used "Crimson Tide" in describing an Alabama-Auburn game played in Birmingham in 1907, the last football contest between the two schools until 1948 when the series was resumed. The game was played in a sea of mud and Auburn was a heavy favorite to win.
But, evidently, the "Thin Red Line" played a great game in the red mud and held Auburn to a 6-6 tie, thus gaining the name "Crimson Tide." Zipp Newman, former sports editor of the Birmingham News, probably popularized the name more than any other writer.

I have seen a few sites that all claim the same story. Here is a link.

The nickname "Crimson Tide" originated with the 1907 Iron Bowl, which occurred during or shortly after a particularly rainy day. Auburn, heavily favored to win, was forced to accept a tie with Alabama after a hard-fought game. Describing the game, one sportswriter described the offensive line as a "Crimson Tide", in reference to their crimson jerseys as they moved down field in the rain.

These two guys above me are correct, but they left out the famous newspaper line that coined the nickname. Alabama is their red jerseys were hitting hard against Auburn, and the line read "Alabama pounded on the Auburn team like a Crimson Tide." And thus, a legend was made.

Crimson Tide comes from the Confederate soldiers from Alabama that when they fought on the battle field they looked like a crimson tidle wave.That said yall are right about when the nickname came about but that is when exactly what it means.

Well, the meaning has changed over the years.

For instance, now it refers to the blood of all the coaches who have been lynched there by halfwit Alabama fans because they are not the Bear.