I'm assuming you mean the regular strelitzia, vs the giant. I have some fairly recently planted ones (1-1.5 year-ish), and the red stem between the green leaf segments seem to be a combination of increased sunlight at this time of year and maybe a bit of stress from them not having a fully developed root system yet. Mine are in a bit of a hotter situation than they might prefer, plus they are not really large enough to self-shade, and there is some hardscape beside them that reflects additional heat. I planted low-growing carrisa between them to fill in and help keep the roots cool, but they needed time to establish, too.

The plants in my neighborhood had a recent flush of blooms, but they have been going into a bit of a rest period within the past few weeks. Do you think the browning of the flowers is to an abnormal extent, or are those flower stalks just finished as they normally would? How large and established are your plants?

Larger clumps will produce more flower stalks, and it can appear as if those plants have individual stalks that last longer, when they are just producing more later-developing stalks and staying in bloom over a longer period of time.

It is a bit better now, and it has a couple of blooms.

I am pretty sure that these dead blooms weren't just flowers that had finished - however, I didn't take pictures or keep track very well. Maybe it just got too cold or something.

Overall, I need to remove a bunch of weeds from it - but, at least it has had some normal blooms. It's a large and old plant. (Well, not huge - like 5 to 6' tall.)

If it's already putting out a few healthy blooms again, I'd lean toward it being temporary stress rather than a disease. This spring had some unusual weather in parts of Southern California, with cool nights followed by warmer spells, and that can definitely affect flowering. Older clumps are usually pretty resilient, but they can still abort flower stalks if conditions aren't ideal. I'd definitely clear out the weeds, since they compete for water and nutrients and can reduce airflow around the base. While you're at it, check the crown for any soft or mushy areas, but if the leaves look healthy and new flowers are opening normally, I wouldn't be too worried. I'd just keep it watered consistently through the hotter months and see how the next flush of blooms looks.