What does log in "backlog" refer to?

Let's start with your definition of log from Online Etymology Dictionary.

"record of observations, readings, etc.," 1842, sailor's shortening of log-book "daily record of a ship's speed, progress, etc." (1670s)

So the log tells what the ship has done. It says what actions the pilot has taken. It records what the navigator has observed.

This definition of backlog says:

An accumulation of something, especially uncompleted work or matters that need to be dealt with: the company took on extra staff to clear the backlog of work

It may be reasonable to assume that backlog is a back-formation from log. The log says what the ship has done; the backlog says what uncompleted work or matters need to be done.

One proponent of the Agile methodology describes backlog as follows:

A backlog is a list of features or technical tasks which the team maintains and which, at a given moment, are known to be necessary and sufficient to complete a project or a release...

The log says what the ship has done; the backlog says what the team needs to do.