One thing that John had learned about Sherlock was what he liked to call 'The Rule of Unexplainable Things'.
For example, Trying out 30 different shades of lipstick? It's for a case.
Melting a bowl of pink marshmallows in the microwave, then a bowl of white marshmallows? It's for a case.
Blue shoelaces in his dress shoes and a green bowtie? It's for a case.
John had simply come to expect that every crazy and -at the time- unexplainable thing that Sherlock did had a means, and in the end it would become clear to him.
However, one unexplainable thing still had John stumped; and this time there was no Sherlock to explain the reasoning, the motivation.
So, John had come to the only conclusion he could, given the little information he had.
Taking a fall from the top of St Barts?
It must be for a case.
And John could only hope that one day, he might find out why.
