This does happen with some varieties of large flowered agapanthus - the flowers are so large and heavy that the stem can;t support them properly. Flower stems sometimes get longer and thinner than usual if the weather conditions aren't great - dull days, insufficient sun or light, delayed flowering, that kind of thing. That said, I know of an evergreen variety in someone's garden and we have to stake the flowers every year, because they all bend over if we don't - and the flowers are huge.

If yours has been in the same pot for 2 or 3 years, you can try repotting into something a bit larger and plant them a little deeper, that might help, and make sure they're in full sun as they start growing.

Were the plants in a little more shade this year? Also, how much potash does the fertilizer that you put on them contain?