Last Saturday (last visit) they were standing proud; a few leaves were yellowing/browning at the top few inches. Today they're all folded over just above the bulb, the stem and everything above it has gone soggy, greater number of yellow leaves. The bulb is fine, nice and firm, no sign of any infection. ???
If the bulbs are nice and firm, they are giving the 'harvest me soon' signal! ;D
Have you lifted any to inspect the part that is in the soil?
I suggest you do this to see if there is any root disease, if not then I would agree with s/sprout