It sounds, from the stripe, like a Miscanthus sinensis. Why not try tying it up for now? Run a stout piece of twine around it, 1-2' off the ground, and tighten it up with a good knot. That should hold it up for the rest of the season.

Then, next spring, cut it back for it's annual haircut, and dig and divide it. That will refresh the clump, and should give you a few years of carefree service.

I just put it out earlier this year,,,,but,,, now that i'm
thinking about it..... I did dig it up from somewhere else
in the yard and didn't divide.
My point is -isn't it going to do the same next year?
I don't remember it doing this when it was in the other
location, but may not have noticed - wasn't in walkway.
What causes it?

It could be caused by many things:

Soil not draining well enough. Even with low water, if it's collecting and sitting around the roots, it prevents oxygen and nutrient uptake.

Soil too rich. Too much compost/fertilizer/organic matter.

Not enough sun.

Critters of some sort messing with the roots. This could be anything from rodents to insects.

Age. Some of these grasses need to be divided every few years to remain vigorous. When you cut it back each spring, check the center of the clump for dying sections. When you divide, take your divisions from the exterior perimeter.

And ... to add to Donn's long and accurate list.

Some grass cultivars are just born floppers or leaners. It is never quite as apparent as when you plant them along a walkway. I speak from experience :o)

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Many grasses are more prone to flopping as they come into bloom ~ Miscanthus sinensis 'Strictus' is a very good example of this.

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Weather can have a big influence. Bad timing of either rain or snow can cause your grasses to flop. The stems seem to be sufficiently tough until the grass extends & blooms ~ then there is an iffy period where too much rain can topple the whole thing. An early snow, before the grass stems have had a chance to 'stiffen' can be equally damaging.

short answer to this long winded reply is: yes, it may just keep doing that.

I have used Donn's suggestion of tying the floppers up with twine. We have had TONS of rain in the last couple of weeks and it all started when my M. sin. 'Strictus' was coming into bloom. Planted, of course, along my front walkway! After all that rain 'Strictus' is still upright and once we have a good solid week of drying weather I am hoping that I can remove its girdle

More on the same subject.
I have an anti-flopping idea that I would like to try next year ~ saw it in a friends garden.
He uses sections of wire fencing - from memory the mesh is approx. 2" x 2" squares. He uses a retangular or square piece of fencing, bent slightly to give a convex shape. This he lies on the top of an already growing plant, ie when it has put on 6" to a foot of growth. I have seen it used on tall sedums, geraniums and anything that has strong stems and a strong tendancy to flop. That's all there is to it. After a couple more weeks of growth the wire has vanished. The idea is that the plant in combination with the wire supports itself. This equals a very natural shaped plant.
I guess it would be a bit like a peony cage but not as limiting and with no legs.

what do you think???

I'll add 2 more cents to the pot...

sometimes its the outer culms of miscanthus that are doing the flopping and cutting back these culms to a 6 to 12" height will eliminate the flopping and the half foot of stiff culms will help bind the rest of the plant vertically.

yes, it is just a few on the outside - i will cut them back ......thanks again