I am curious about the planned move of the US embassy from Tel Aviv to Jerusalem. Why the urgency and what is the real reason behind it?
Why the urgency
There are two answers to that:
The move is not a drastically new idea. If anything it's behind schdedule, literally:
The move is mandated by Jerusalem Embassy Act, passed by Congress in 1995, 20+ years ago, and moreover, officially setting the deadline for the move to be in 1999.
Both Presidents Bush and Clinton promised to move the embassy when campaigning as candidates, same as Trump (source: CNN). Nothing new here.
But seriously, why more of an urgency for Trump? Seems he's more serious about it than Bush or Clinton.
We don't know the answer to that for certain, but the following factors are clearly possibilities that might influence him:
Trump is positioning himself as anti-Obama in general.
That involves a lot of policy positions, and Obama's, to put it mildly, not-very-supportive-of-Israel position is one of them.
Trump is positioning himself as a leader of Republicans in general, as all presidents do of both parties.
In case of Republicans, support for Israel in general, and Jerusalem embassy move, are a very popular position, strongly reinforced by evangelical views. So, this issue is a point scored by him as far as shoring up support from Evangelicals and republicans in general.
“Evangelical Christians always have Israel on the top of their list,” - Kellyanne Conway in Hugh Hewitt interview.
Trump might be courting Jewish and other Pro-Israel Democratic vote, in light of the fact that Obama and ever-more-progressive DNC allowed themselves to make it a possible wedge issue.
“It’s something that our friend in Israel, a great friend in the Middle East, Israel, would appreciate and something that a lot of Jewish Americans also have expressed their preference for.” (source: same Conway interview)
While the efficacy of such a move is questionable in US domestic politics, it might have an effect, especially since:
It has a chance to effectively disarm baseless accusations of anti-Semitism that the Left layed on him during the campaign.
Conventional wisdom is that this is a futile effort, since most polls indicate that domestic political preferences are of stronger importance to American Jews than Israel is. But then again, Trump did manage to make a mincemeat of conventional wisdom a number of times during this campaign :)
Seriously, what urgency? This is Trump!
One thing you need to take into account is that Trump hasn't done anything specific yet (not having been inaugurated) and that he has already walked back a large portion of his campaign promises.
Specifically, just 1-2 hours before you posted your question, news reports were that Trump is now considering watering down this stance, and merely housing US Ambassador in existing Jerusalem US consulate instead of moving the US Embassy.
what is the real reason behind it
Nobody knows the "real" reason since nobody overheard Trump discuss this in a situation where he thinks he's in private. Absent Wikileaks type leak of confidential communication, or "hot mike" type incident, you're unlikely to find out the "real" reason, as opposed to subjective opinion of people speculating on what the real reason is.