The good news is that with Smith's departure being several months out, Intel has ample time to try to fill the role (though I strongly suspect that Intel executive Diane Bryant, who is currently on leave, will get the spot).
The bad news is that yet another key Intel executive is abandoning ship.
Though only Smith (and possibly a few close friends at Intel) knows why he's leaving, and since we're unlikely to ever find out, we can only speculate as to why he's leaving.
Here are two potential reasons that I can think of.
Smith may want a path to the CEO role
Smith was reportedly one of the contenders for the CEO spot at Intel when former CEO Paul Otellini abruptly announced his plans to leave the company in late 2012.
The selection process lasted a while (Otellini announced his retirement in November 2012, and his successor was named in May 2013), but ultimately Smith and others were passed over in favor of Brian Krzanich as CEO and Renee James (who is no longer with the company) as president.
In the aftermath of the Intel CEO selection process, longtime Intel executive David Perlmutter – another potential candidate for the top job – left.
Perhaps Smith quit because he aspires to be a CEO and doesn't think that it's likely that he'll get to do that working at Intel.
Not only would he face significant competition for the role, should it open anytime soon (of this, I'm doubtful -- but the reasoning behind that will be for another time), from internal candidates, but I'm sure that there'd be plenty of competition from outside of the company for it as well.
Smith may want to leave on a high note
Another possibility is that Smith wants to leave Intel on a high note. Keep in mind that Intel's financial performance continues to be quite good. The company is now guiding to north of $61 billion in annual revenue for the year, and business right now seems to be going well -- the PC market is performing better-than-feared, the data center business is chugging along, and Intel's investments in the Internet of Things and non-volatile memory technologies seem to be paying off, too.
If Smith thinks that now represents "peak Intel," or that we're close to it, it could make sense for him -- particularly if he wants a CEO position somewhere else -- to leave the company on a high note, rather than wait until things look appreciably worse to leave, tarnishing his track record and reducing his opportunities.