That particular span of time was one of relative calm. That isn't to say fighting wasn't happening, but due to coincidence, it was a period where on most of the major fronts major operations had paused, or were winding down.

Looking to the Eastern Front, operations around Kharkov ended in mid-March, and operations at Rzhev had concluded by the end of the month. The spring "Rasputitsa", when the snow melts and the roads turn to mud, meant that both sides were not inclined to mount new offensives over the next few months, which were instead spent mostly building up the respective forces in the area of the Kursk salient, which would see the Germans begin offensive operations once again in July. As such, while clashes of course happened throughout the front, from the beginning of April through the first week of July, the German-Soviet front -where the plurality of casualties in the war would occur - was in a relative state of calm, helping to explain a good deal of the dip you are seeing.

This wasn't the only such case though. In the fight between the Western Allies and the Axis, there to a significant pause occurred in roughly the same period. Through early 1943, the British and American forces steadily pushed the Germans and Italians back into Tunisia, and the Axis would hold out another month but finally the entire force in North Africa surrendered in mid-May. This was the major theater for ground operations at the time, and thus with the campaign ended, there was little fighting to speak of there until Operation Husky saw Allied troops landing on Sicily in July. So likewise, from the latter half of May through early July, there was considerably little action going on against Germany.

I'm not familiar enough with the war against Japan to detail the specific lulls on the island campaigns, New Guinea, Burma, and/or China, although off hand I would note that Guadalcanal wrapped up with the Japanese evacuation in February, and I don't believe the US conducted major landings until that summer, but on the whole I would mostly leave that to someone else to fill in, but notable lulls happening concurrently just in the two fronts detailed above are more than enough to account for such a dip. Unfortunately there isn't anything particularly interesting in explaining why it is the case, but certainly coincidence in of itself does have quirks worth noting.