Why does my lemon tart filling sometimes curdle?
RockyFord's comment has the heart of the matter, I think--the acid in lemon juice will begin to denature or curdle to the proteins in the eggs if it comes into direct, undiluted contact with them.

You can minimize this effect by beating together all of the ingredients except the lemon juice prior to mixing in the lemon juice.  This will add a lot of sugar and milkfat which will buffer the interaction between the citric acid in the lemon juice with the egg proteins. 

You can then bake your tart as normal.