Beer darkens in secondary. Why?

[Soulshine]

So this has happened past few batches now. Could it be bc I'm doing a 5g batch in my 20g fermenter? Too much head space?
Most recent batch did a very pale ale with 1/2 gallon pasteurized cucumber juice added to secondary. Poured first pint today. Taste good but it went from straw color to yeungling like color. 1) guessing still good to drink since it actually taste very good 2) what the hell is making my pales turn darker with age? Frustrating the hell out of me. Happened with my lager batch before as well.

Thanks for any help.,.

 

[jekeane]

I am sure part of it is more yeast dropping out of suspension thus removing the white haziness the would add to the color.

 

[BeerBDrew]

Darkening color is one effect of oxidation. The 15 gallons of head space is your culprit, though the addition of juice adds another variable.