Great question, I have the same problem sometimes!
The Tea Association of the USA has this to say...
when tea steeps, tannins - the natural compounds that color tea leaves - are released into the boiling water. The heat helps dissolve them, and the brew is clear enough to see through. But refrigeration causes tannins to separate out again, turning the tea murky. Generally, higher-quality tea contains more tannins (because it is richer in solids) and is more likely to become cloudy. Hard water is another culprit behind hazy tea.
