When good broths go bad

You all know about the broth bag, right?  You save all your vegetable scraps in a ziploc bag in the freezer, and when it's full you use it to make stock.  Well, I've had a bag in my freezer for a while now, and I want to make some bean and barley chowder this weekend, so I decided to make some broth.  I put the contents of the bag in my biggest slow cooker crock, filled it with water, and left it on High all day.

When I got home from work, I could smell the savory broth the moment I got off the elevator.  It smelled great, and my apartment smelled even better.  I turned off the slow cooker and strained the broth into a storage container.  It was a lovely dark brown, and smelled like onions and other wonderful homemade goodness.

And then I tasted it.  It was bitter.  I tasted it again.  Still bitter.  I tasted it a third time.  Did I want to use this bitter liquid as the base for my chowder?  I did not.  I poured it down the sink.

I'm trying to figure out what went wrong.  Here's what was in the bag:

onions (ends and skins)
garlic (same)
carrot peels
potato peels (just a few)
celery tops and ends
fennel tops
lime rind
squash ends -- summer and winter

I'm thinking either the squash or the lime rind was a mistake.  The other stuff seems benign.  I always use the onion skins in my broth because they make it a pretty color, and I can't remember ever having a problem.

Any ideas?  I had both red and yellow onions in there, but I don't think that can be it.

So I will be making my soup this weekend with Spike broth powder, plus about a cup of bean stock I have languishing in the freezer. Gayelord Hauser was a charlatan, but the broth powder is pretty good.  I'm going to grind up some dried wild mushrooms too, as recommended by America's Test Kitchen, and hope that does the trick.