Put simply, it was all because of wartime shortages.
"The period of the increase was due to war regulations. The lack of raw materials increasingly forced gravities to be reduced. In order to save malt and barley, it had to be constantly stretched. With the reduction to 6%, there were no difficulties with bottom fermentation with regard to yeast propagation, maturation and fermentation. When, according to further regulations, beers with a wort content of more than 5% were not allowed to be produced and the breweries were forced under the constraints of the circumstances to lower the wort content to 4%, then to 3% and finally even further, bottom-fermenting yeast started to fail. It hardly supplied the required amount of spores for its own business, sat down hastily without having gotten into sufficient fermentation and lay loosely on the ground."
"Obergärige Biere und ihre Herstellung" by Dr. Franz Schönfeld, 2nd edition, Verlag von Paul Parey, Berlin, 1938, page 131.
I'm not surprised that bottom fermenting yeast struggled in such watery worts. One solution I've also seen practised in the UK during WW I.
"Although it was possible for various companies to obtain the required amount of yeast by growing them in stronger worts, this option was not open to others.
Under these conditions, top-fermenting yeast could now be adopted to do emergency work. It could tolerate the poor nutritional conditions in thin worts, had the ability to reproduce so abundantly that not only was enough yeast harvested to continue, but that there was even enough that it could be sold to other firms. It also matured and did not easily degenerate.
So breweries often turned to top fermentation; also because, when using top fermentation, they were able to meet the increasing demand for beer to an increased extent by using sugar and sweeteners.
As long as restrictions were imposed on the use of raw materials by law, top fermentation remained at a high level.
When the restrictions on the use of barley and malt were relaxed and breweries were able to brew stronger beers, they turned away from top fermentation and returned to bottom fermentation for the production of lager beers. Top fermentation had done its duty. It could be switched off again."
"Obergärige Biere und ihre Herstellung" by Dr. Franz Schönfeld, 2nd edition, Verlag von Paul Parey, Berlin, 1938, page 132.
I've seen UK brewing records where ther's the odd much stronger version brewed, which is then blended with weaker beers post-fermentation. The point being to get healthy yeast to be pitched into later brews. And that was when worts were in the 1020ºs, considerably higher than the 3º Plato (1012º) they had been forced down to in Germany.
Weirdly, the Reinheitsgebot seems to have remained in force for most of WW I. By top-fermenting, a wider range of ingredients was allowed. Including sugar and artificial sweeteners.
But it was all only temporary. As soon as brewers could produce decent-strength beer again, they returned to bottom fermentation.