This a common problem with these dwarf citrus plants. They are very 'hit and miss' with their husbandry. They need a constant temperature and are either too wet or too dry; suffer from mildew or red spider mite. They do look very inviting when viewed in the garden centre with their minature fruits but, in my opinion, not very suscessful subjects to grow. I am sure someone will say they have one that's been growing for years but I would say they are in the minority.

perhaps it is now too dry, give it some water and see if that helps. We have 2 that were grown from pips, one is looking sad the other ok.

I have a large grapefruit grown from a pip, and it also loses it's leaves this time of year. I always assumed it was sort of Autumn style leaf fall but being evergreen it does it in spring before new growth.

Never seems to have harmed the tree so far