The doldrums are more politely known as the Intertropical Convergence Zone and in maritime lore, they are the period of the voyage where the ship drifts in dead air and still seas. In the doldrums, one waits (and waits, and waits) for the wind to change or a current to pass, and then leaps to action to make the most of the gust or ripple to edge a little further onwards. In the mean time, the ship drifts hither and thither, turns stem to stern, and everyone gets pretty surly.
You can see where this makes a nice metaphor for the labor market in libraries, archives, and museums.