Could be water, too much or too little. Could also be virus.

Any chance of images?

See one of the Euphorbia Fireglow in foreground with the withering top. Note the dying thyme to the right. If it's a virus, what do I do?

The thyme looks like it got too wet. The spurge looks like the older leaves have mites or insects, maybe there is a buildup of them at the tip that has resulted in it being hit harder. Otherwise, maybe there are chemicals or pathogens in the soil that are damaging your plants.