Without even looking at the pic I was about to answer heat scald from a recent sudden hot day or two...my hydrangeas always seem to wilt badly and even permanently singe edges of leaves on the hot low humidity days, wherever they are exposed to lots of sun.
However, that damage pattern is not what I'd expect from heat/sun scald...you guys may be onto something all right...
No, if anything such damage would be too much fertilizer. It might happen to need fertilizer as well, but lack of fertilizer would not customarily result in scorching - unless, perhaps, there was some weird trace mineral deficiency that sometimes produced this effect.
Down here a good idea would be to clip an affected stem and take it to a Cooperative Extension office or Master Gardener clinic. Maybe the Province offers some kind of similar service.
It looks to me as if it may have scorched/scalded in the 90 degree + days we experienced a few weeks ago in the northeast. It would only take one day of 90 degree plus weather and a drier than usual rootball for this to occur. I would prune the dead out and continue on a regular watering schedule. Endless Summer is a pretty tough plant. We have to keep them under a 30 percent shade cloth at our nursery or they wilt down in the hot afternoon sun.