Is it OK to inject voltage to an unpowered LDO output?
As Ignacio pointed out there is a parasitic body diode inside the regulator from the output to input. 

If VBUS has a very low source impedance then the C15 will get charged quickly through U5 and there is not much limiting the current. Or, anything connected to the VINLDO input could draw current from VBUS through U5 (for example, shorting to ground could kill the regulator).  

Edit: To make this a bit more clear, here is the internal diagram of U5: 



One possibly practical solution if this appears to be an issue is to add a Schottky from output (across U5's body diode and in the same direction) to input and a polyfuse in series with the VINLDO input.