Facebook's Libra - Why would it vary in value
It will vary in value because trust in it will fluctuate just as trust in facebook fluctuates.

There will also be volatility as trust varies in confidence in the payment system; its integrity and security; how exchangeable it is; its exposure to fraud,  to tax, to money-laundering investigations

For holders, there is also significant counter-party risk in whether Facebook chooses to honour their holding. And the majority voting rights in Facebook are vested in a single person. So their counterparty is, in some regards, essentially one man and his whims.