Hey mate,
Congrats on the purchase. Can't go past a PJ!
My guess is it's chlorophyll withdrawl. Kwong mentions it in his book on Ficus bonsai in Temperate Climate. The thing to note is that it's not a problem. PJ leaves have about a two year life span before they need to be replaced by the tree. Now is one of the times when they drop their leaves. Several of mine are doing the same thing.
The other option is a fungus, and the more experienced growers may be able to say that with more certainty - although I'd put my money on the chlorophyll withdrawl.
Enjoy the fig
