Leaves Dying on Avocado After Transplant?

I have two avocado trees, one is one year old and the other is 2 years old. The containers where I have them planted are getting small and I decided to transfer them to another bigger container and after a week their leaves became brown and they are dying. Can you tell me why? and if there some way I can recover then before they die. I have to mention that I'm living in Osaka Japan and here in winter is a little cold.

Thanks

Luis