Apple tree experts,

My parents have a SnowSweet apple tree, and in its 3rd year it put out a nice drop of about 36 apples.
After the harvest about 1/2 of the tree had purple leaves.
Dormancy came and went with no signs of disease or damage.
We pruned the tree in early March.
This spring the tree was afflicted with fireblight pretty heavily, about 20% of the tree had to be pruned out.

Has anyone dealt with this type of behavior/disease before?
I was under the impression that purple leaves is a sign of overly wet roots, they do have clay soil.
I am also under the impression that SnowSweet is a fireblight resistant variety.

Thanks for your insight.

-Eric