I really don't know the reason, I've heard too much shade or too fertile soil may cause this, although I have mine in full sun also and they flopped and I doubt the soil they are in is too fertile but maybe it is. I have 3 variegated Miscanthus and all of them flopped the 2nd year. After that 2nd year it never happened again. I tied fishing line around them. It held them up beautifully and was also invisible. Hope this helps.

Sounds like it might be Zebra grass (Miscanthus sinensis 'Zebrinus' which is supposed to flop over....

If you are wanting a truly upright variegated Miscanthus, go with Miscanthus sinensis 'Strictus' (Porcupine grass).