Because the toilets at the White House were eco friendly toilets that required them to be flushed 10 to 15 times to get classified documents to go down them.

And once Trump had the boxes at Mar a Lago Trump never got around to finish flushing them all because he is fat and lazy.

Basically, he flushed classified documents down the toilet at the White House but once he had the documents at his home in Mar a Lago he put off flushing the remainder of the classified material he wasn't able to properly flush at the White House due to low water consumption toilets that took 10 to 15 flushes to get classified documents to go down and yet they still clogged the toilets.

Obviously toilets were a big problem for Trump at the White House. Now we know why.