During the time when humans have been hunting elephants for ivory there has been a significant trend to get smaller tusks and it looks as though what's happened is that poachers and legitimate hunters have been all the time shooting the biggest tuskers to get the ivory – with the result that there was a massive selection pressure in favour of smaller tusks.
So the average tusk size has been going down and down. It is one of the spectacular examples of evolution happening before our very eyes within living memory – it can happen very, very fast.
I love Kipling's Just So Stories, 'Oh Best Beloved', but they are shockingly un-Darwinian – the Elephant got its trunk by being pulled by a crocodile!? Of course that isn't how it happened, natural selection works in a totally different way. So it's nothing like being pulled or pushed from outside.
As for what the actual selective pressure, what was the advantage of having a long trunk? Well one possibility is that it was something to do with drinking.
There were other reasons why elephants became big and tall – reaching the tops of trees for example like giraffes. And like Giraffes that raises problems for drinking.
In the case of the Giraffe the whole head has to go down, and that means the head has to be small. Elephants did it in a different way – they kept the head large, which has some advantage, and they then have a long pipe leading out of the head which is the trunk.
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