I use the One Step no-rinse sanitizer, so it's probably not that. I do soak my carboys in bleach water before use but always do a thorough rinse followed by a soak in One Step. But if the problem came from my carboys I wouldn't expect the original samples to have turned out OK. So I'm wondering if maybe I didn't sanitize the bottling bucket well enough.
Probably a long shot: I know One Step is supposed to be no rinse, but I wonder what happens if you don't thoroughly dissolve all of the powder and end up racking on top of some undissolved One Step. White powder in a white bucket...I could have missed it.