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.