Pine needle drop

If should not have affected the tree if its a mossicide as they only target moss. Normal weedkiller would have possibly affected the tree esp. something like sodium chlorate. Roundup wouldn't harm the tree as run off from the paving (unless you got some one the tree) but roundup wouldn't kill moss.

Another possibility is evergreen trees drop their leaves/needles slowly all year (my holy drops them around may/june) but the recent hot weather may of encouraged it to drop more.
It is quite a mature tree and i doubt that it would be that badly affected unless it was from something like sodium chlorate which is evil stuff!

Depends what you used to kill the moss. It may say on container what its made up of and if or not to get it on cultivated plants.

Sodium chlorate is used to clear uncultivated areas of plants - you may find the tree dies.

Sodium Chlorate is a total weedkiller but you must have used a heck of a lot if it has indeed killed a twenty year old pine.
The weedkiller might have upset the pine and increased the annual needle fall. All you can do at this time, Busman, is to wait and see how the tree progresses.