Why is Germany re-patrioting its gold?

First answer, why not?  It's not like the USSR is going to invade.

Second answer, in case it needs to rebuild its currency:

Central banks, investment funds, and individuals hold gold mainly as a hedge against volatility in currencies or in financial markets. Lately, the principal interest in gold has been as a hedge against inflation. But it is also useful for other things: For instance, having very large gold reserves is a handy thing indeed if you should need to construct an entirely new currency from scratch, as Germany may very well be obliged to do should the euro disintegrate. France could have to do so as well — which the cynical mind might see as a good argument for Germans to keep German gold in Germany rather than in France. It is worth keeping in mind that while we are all good liberal democrats now, Germany and France have been at war within Mick Jagger’s lifetime and twice in John Gielgud’s. Ancestral mistrust does not evaporate in one or two generations.

As the endless euro crisis continues to inflict damage on the German economy, the Germans must certainly be asking themselves what exactly they get out of the euro, and out of “Europe.” While there are some appreciable efficiencies from the reduction of foreign-exchange costs and trade benefits from the reduction of exchange-rate uncertainty, most of the economic benefits Germany derives from European economic and political integration could be had with nothing more exotic than free trade and liberal immigration rules. During the lead-up to the euro’s debut, there was endless discussion of the single currency’s benefits. And there have been benefits. But there have been costs, too: very high costs. If the Germans should decide that those costs are too great to bear, Berlin is going to want those gold reserves at hand.