According to that article, there seem to be two commonly used explanations for why he did that. One of them is that he was showing that he could, and the other is that he was warning his listeners to be like that tree. I'm definitely not the most qualified person to say which is right, but I would lean towards the second because in addition to it fitting better with the rest of what I remember the New Testament saying about him, he also apparently told a parable that was almost exactly the same and pretty much always interpreted interpreted that way, so he seemed to like that metaphor.