Why is ATV-3, Edoardo Amaldi, prepared for undocking today?

During today's EVA-31 by Russian cosmonauts Gennady Padalka & Flight Engineer Yuri Malenchenko, the ПхО compartment (PkhO: Transfer compartment Переходный Отсек Perekhodnyi Otsek).

The PkhO connects three of the Russian ISS modules and is used for docking – will be de-pressurised.

As a result, the Russian Service Module (SM – to which ATV is attached) will be isolated and unmanned; all hatches in the Russian segment of the ISS are being closed for today's EVA.

Since ATV must at all times be able to perform an off-nominal undocking, it was necessary to prepare Edoardo Amaldi for undocking in the unlikely event that an automated undocking (i.e. without crew intervention, since no one will be in the SM) were to become necessary during the 14-hour period of hatch closure.

This is simply a precaution because there is no reason why ATV-3 should undock; the preparations are purely based on ISS flight rules.

During today's spacewalk (set to start at 16:40 CEST and run ~6.5hrs), two of the ISS crew will be in the Russian Poisk (Russian: По́иск; 'Search') module, also known as the Mini-Research Module 2, two will be in US segment and two will be out on EVA.