Your onions are just being onions and are completely normal IMO. When day length triggers a onion to bulb, the energy stored in the leaves is transferred into the bulb and the leaves yellow. The bulbs develop very fast over a period of 3-4 weeks. Eventually the neck will soften and the leaves will flop over. I harvest about 1 week after the flop. Stop fertilizing!

Actually those are real nice looking onion plants, especially considering how little actual soil you have them in. Acting pretty much normal it seems to me, too. Like Jack says quit fertilizing and I say quit worrying. I tend to want a quite a bit of drying on the stems after they fall over and a nice set of husks on the actual bulbs before I harvest them. The bulbs will be right at the surface so you can watch them develop.

Thank you both for the input, I kind of figured I was overreacting here but I just wanted to be sure with how long these have been growing for. Checked on them today and they seem to be growing fine still, with some of the bulbs getting close to 3" in diameter now. My only other concern is that I have three of these beds, and only two of them are bulbing up. They are all near the same location, have the same soil and get the same amount of water, yet the last bed doesn't appear it has started bulbing at all yet. They are all long day onions, but do certain varieties just take longer to start forming bulbs?

Edit: Looking over my notes, I believe the last bed is the intermediate/day-neutral variety "Candy", which I was told would still bulb up in southeast MI. Will these just take longer to bulb or is this bed a bust?

I agree with the two comments above. Those are great looking onions, you are doing good. Don't give up on the Candy onions, just wait until the tops break over and see what you get for bulbs, that's the only way to know.