What is interesting from the two charts above is that while the numbers of battles have dropped, the number of transactions has remained the same, meaning there are other type of operations that have increase, most likely the renting cards. This especially with the third-party services that are offering cards management and are making a lot of renting operations.

If I take a look on my rental income, it decreased recently.
The number of battle and active players were also decreased as your chart, so I have a guess that the renting operations were also reduced.
And I think a huge number of transactions comes from claiming battle/chest SPS rewards and LP reward