Why did my mulberry’s branches die?

[Gabitule]

I bought a dwarf mulberry 3 years ago (Charlotte Russe). I planted it in a large pot in a south facing location. Last year it produces a handful of fruits so I was looking really forward to this year.
This spring I noticed that, although the branches had tiny leaf buds, these were brown and crispy and dead! I cut the branches and they were dead. The trunk seems ok and it has a couple of leaves sprouting straight from the bottom of the trunk, but all the branches were dead.
I am completely at a loss as to why they have died. I cut all the branches off as I assumed whatever is affecting them could spread to the tree’s trunk but, to be completely honest, I have no idea what I am doing.

Does anyone know why the branches died? is the tree going to recover?

Mulberries are meant to be frost hardy. Even if this dwarf variety wasn’t that resistant to frost, why did it survive last winter but not this winter? Plus, it survived long enough to make leaf buds. And why did ALL the branches die?

Its all very sad :(

Did it have any sign of disease at all to cause the limb die back? When you cut into it is there any reddish colour inside the branch?

Plants in pots do still need watering in the winter months if it hasn't rained much, and we did have several periods early this spring when it hardly rained at all for a couple of weeks. I started watering my pots earlier than usual this year.

The other possibility is vine weevil attack, and they have eaten a lot of the roots.

Edited as I forgot to quote the message I was replying to

No red colour inside and no signs of disease. Tonight I noticed that the tree ‘stumps’ (there are a couple of sections left) were leaking. I wonder if this is because I watered the tree yesterday and the tree has taken in too much water. I feel a bit helpless, I have no idea what to do to help it.
I’ll take a photo tomorrow and send it

Thank you. I did water it a bit in the winter and more this spring. One of the my thoughts was that I might have watered it too much, but I don’t think I did. I just don’t know.