I have noticed this on multiple occasions after another round of translations had been announced.
Translations strings for German get marked as ‘Needs work’ for seemingly no reason (no source string changes, no actual issue with the particular translation).
I don’t know if the behavior is linked to changes of source strings in other places. It happened for the most recent Translation Round 2 as well.
Depending on the amount of affected strings this can be quite an inconvenience. The way Pootle handles things one has to fix all strings of this sort among the ones which really are incomplete.