Holly tree struggling

Hi, new member here and novice gardener but very keen.
I have a problem with my Holly tree, it lost all of it's leaves during the first few months of the year. It has been in a large plastic pot for the past three years and thriving at four feet tall, it even produced its first berry last year. The leaves gradually turned black and fell off and i just noticed recently that some branches have also turned black so I took the bold step of cutting all the black bits off and chopped about a foot off the top and given it a fair trim around the sides. I transplanted it into the ground a few weeks ago as I have just replaced all of my hard clay soil with a nice new topsoil/manure mix and it is starting to leaf again.
What should I do next? What do you think caused this and could it happen again? Could it be that brutal winter, under-watering or did it just knacker itself out squeezing that wee berry out?