There are many possibilities, so I'll go through them all:

1. Some developers simply go dark, even if well stored. Ansco 130 is prone to this, as is Rodinal and its cousins.
2. Most developers need to be stored away from oxygen when possible. This is usually done by keeping bottles full. You can keep the bottle full by using a collapsible bottle, decanting the liquid into smaller bottles, inserting relatively inert nitrogen gas into the jar, or inserting glass marbles into the developer jar to displace the air space. The more air that's in a developer's bottle, the more likely it will oxidize. (Many developers go dark when this happens, although some, like XTOL, do not. Some developers, like PMK and Rodinal, do not suffer from reasonable exposure to air.) Also note that some plastic bottles have leaks, and you will get more air interchange as well if the cap is not tightly closed.
3. Developers do not like light. Direct light, especially sunlight, will degrade many developers.
4. Developers do not like heat. They should be stored in as reasonably cool conditions as is practical, but above freezing. Anything in the 10-to-20-degree range is fine. Life will be shorter if you store at warmer temperatures.
5. Developers exhaust. They have a finite capacity. If you use them to their limits, they will darken as you go. The developer's instructions should give you a sense about capacity. Note that capacity varies by dilution, if the developer is diluted from a stock solution.
6. Although this wasn't a factor in your situation, age darkens developers. The longer you store them, even if perfectly, the darker they will tend to get (with some exceptions, again, like XTOL).

You didn't specify what developer you lost, or how you stored it, so it's difficult to give you more specific information at this time.