What would have caused my wort to darken in the fermenter?

Experimenting with a recipe for a Scottish ale, from extract. On brew day, my color was just a little lighter than I anticipated. The color darkened slightly during a very active fermentation. (It blew the air lock once.) As fermentation slowed, and even more now that it's in my secondary, the color is considerably darker. Is it normal for the brew to darken significantly during or after fermentation?

Steeping grains:

• 1 lb medium crystal (60L)
• .5 lb "crisp brown"
• .375 lb pale chocolate
• .125 lb peated malt

Fermentables:

• 7 lb Light DME
• 1.5 lb light brown sugar

Hops:

• 1.5 ounces East Kent Goldings (5.8) - 60 minutes
• .5 ounces Fuggles (4.2) - 10 minutes

Yeast: Wyeast Scottish Ale #1728 (two smack packs)

Thanks!