Thanks for the question, not sure about why the branches are dropping off after such a short period of time, maybe its been going on longer than you realize. is there any bark being stripped off these branches, the grey squirrel always does this, here is my answer below. if this answer helps you please click on the like button or leave a comment on one of my hubs:-)
Type of Agent
Host and Localisation
On leaves and sometimes young shoots
Appearance of small yellow blisters on the leaves at the start of the season, which then turn into dark spots. the centre of these dots turns progressively grey. An early leaf fall at the end of the summer can occur in a severe attack.
other fungi causing anthracnose, such as Cercospora microsora, Septoria tiliae, Apiognominia tiliae
all dead and dying branches should be pruned out of a tree (always), remove and destroy any affected parts including fallen leaves. if using chemicals a thorough check of regulations for your area should be made or consult a qualified arborist.
browning of the leaf margins and discoloration could be
hope this helps