The reason is based on the distribution of electron density within the intermediate radical anions that appear throughout the Birch reduction. I personally use the next reactions as guiding examples:
Electron-withdrawing groups stabilize electron density in the ipso and para positions through conjugation so the negative charge will mainly be located during these positions subsequent protonation occurs para .
Electron-donating groups destabilize an adverse charge (in comparison with simple benzene) through conjugation which effect could be most powerful in the ipso and para positions.
Thus the negative charge will mainly be located in the ortho and meta position.
@Marko Should you have a similar para*/*ipso pattern for any electron-donating group (EDG) as you've to have an electron-withdrawing group (EWG) you'd convey more electron density near the EDG or in the para position. However the EDG includes a free electron pair, i.e. a higher electron density, which within this situation might be effectively participate in conjugation using the electron density in the ipso and para positions. And, loosly speaking, when two high electron densities interact they repel one another thus raising the power. Philipp Sep 6 '14 at 13:45
@Marko Using the ortho / meta pattern, the orbital interaction and overlap differs and also the two high electron densities can better walk out one anothers way. Within the situation at hands it is quite hard to show this when it comes to resonance structures. Orbital pictures could be more appropriate. Philipp Sep 6 '14 at 13:48