Maple decline is likely due to a combination of factors, including some different species of opportunistic fungi. These fungi infect a tree that is already stressed. Weather patterns over the last few years have stressed many trees, and the cool wet spring created an environment where these fungi thrive.
The City of London is actively monitoring maples on City owned lands (including road allowances and parkland) and pruning or removing trees as appropriate.
Protect the trees on your private property by pruning dead or dying branches during dry periods, to limit the spread of the fungi.