Why did this happen?
Is this bad? Aside from wasting beer, it seems to me that this would invite infection? If it means anything, I smelled the airlock and it seemed mostly vinegar, but tasting the airlock (and spitting it out) revealed no vinegar taste, more of a terrible beer taste.
Background: This is my fourth batch of extract brewing. This is the second time I used an aeration stone. I pitched the yeast on Sunday around 4pm. I believe it was fine this morning (Tuesday) at 8am, although I might not have actually looked. At 10am it was overflowing. I followed the kit instructions perfectly and had 5 gallons of wart in the 6 gallon bucket.
I sanitized a new airlock and replaced it.