...may be because the Captain has delegated duties in the cockpit and while the F/O and himself were dealing with what was at the time a critical, but not yet a distress situation he asked the relief F/O who often seats in the cockpit to send an ACARS message to the company related to the encounter of severe turbulence... He might have had compelling reasons to do it but not enough elements are available at the present time to fully understand his decision. If memory serves, it seems that the chronology of the events shows that this message was sent early on, minutes before the faulty and more critical messages were automatically sent. That is only a scenario among hundreds of possible ones.