This can happen from frost damage. But it is also often a sign perennials need to be divided. Agapanthus seems to need division every 4 or 5 years. Cut you clump into several parts and you will have even more next year. The roots will be tangled and it's a bit of a pain but worth it.

Thank you. How come this one's leaves turned yellow?

Did it get regular water? Any chance of herbicide damage from lawn?

Is that last plant even an Agapanthus? Looks more like a Phormium to me. If so it's meant to be that colour.

No herbicide exposure. How often and how much/how long should I water agapanthus plants?

Igor, I am pretty certain the yellow plant is not an Agapanthus and has nothing wrong with it. If you look you will see that the leaves are much more sharply pointed and if you feel it the leaves will be stiffer and sharper unlike the Agapanthus' soft fleshy leaves. Also it is entirely evergreen. Phormium

yes, I agree - agapanthus leaves are always rounded at the ends - never growing into a point...although I have evergreen and deciduous agapanthus.