Originally Posted by Rob
A couple other things it could be.  First, stress due to lack of water or excess heat (mentioned previously).

But one thing that was mentioned that I have seen in my plants:  It can indicate that the tree needs up-potting and/or up-potting.  There comes a point when you can fertilize all you want but it just needs more mix to grow in so that there is enough root mass to support what it wants to do up top.  Or the roots could be circling around so much that they just aren't efficient enough to provide the needs of the tree.  This then exacerbated during a hot day when the outer edge of the pots, where a large percentage of these roots now reside, is heated and dried out. 

If I see persistent yellowing up top that doesn't go away after liquid fertilizing and/or increased watering, I know there's a good chance it's time to take that guy out, cut off those circling roots, and put it in a larger pot.  It's not ideal to do this in the summer, though.  Unless it's really bad it's best to wait for the spring.  Sometimes I do it in the summer anyway, but you have to baby the tree for awhile after that until it recovers.