The second plant is commonly known as Mock Orange, not Mexican Orange, Macpat, and its proper name is Philadelphus (Mexican Orange is often used in reference to Choisya ternata). I suspect the plants have been exposed to some kind of weedkiller, either by spray or up through the roots. I hope only these few branches are affected.

Thanks Bamboo. I have asked my neighbours if they have applied weedkiller but they say no. Last summer, my neighbour applied strong weed killer to her gravel garden and after heavy rain, my border adjacent to her garden lost a lot of plants...my garden is a foot lower. I know that she hasn't applied weed killer this year but am wondering if it is leftovers from when she did apply weedkiller the year before. I will keep an eye on the plants and hope that they will survive.