While mating, they disappeared under the rock, surrounded by plants.
What trigered it, I don't know! Last weekend I was away at an Aquascaping workshop with George Palmer (UK) organized by NADA (Nature Aquascaping Design Antwerp) in Belgium, so I only fed them last on Saturday and again on Monday. Normally I do water changes every day of 30-50% but didn't do so this week. The only thing other that was different this week that I overfed a bit on Monday & Tuesday with earthworms, more ramshorn snails and cut clam meet. Today they weren't fed yet.
I have 4 and all are in the same tank.
I'll try to keep posting some updates.
You can see my setup by clicking the link (pic) below.

