There is a lot of argument about that incident in the "Jesus was not supernatural but he existed crowd". A few main solutions:

1. It was understood that the next Messiah would build the 3rd temple, but you can't exactly rebuild the temple if there is a temple. So he was trying to bring about the events.

2. Roman coinage was dicey for strict monotheistic people to use hence the need to change it before you entered. It was a sore point for the holier-than-now crowd. Oh you use forbidden currency normally but change it at the temple? Morality when it suits you.

3. The temple had a dual-aristorcracy structure. The outside was run by one and the inside by another. The outside was more politically acceptable to attack. It definitely wouldn't have been the first time one of the other Jewish factions had gone after how the Temple was run. By attacking the outside one he could set himself up as the quite a few "restorers of the Temple".