[DD.]

Sorry Prakash, you have this one wrong.

I've seen them at it, it's a favourite game for blackbirds especially. You can find unrooted onions feet away from where you put them in.

[Salmo]

You are both right.

Blackbirds pull them out, probably to see if a juicy worm comes up with the roots.

The sets put out roots and if the set is loose in the soil it will often push itself out.