Sounds intentional, probably something sent by the USB host controller driver. I guess some hub chips might send it independently as well in some scenarios. US

Sounds intentional, probably something sent by the USB host controller driver. I guess some hub chips might send it independently as well in some scenarios. USB bus resets (both D+ and D- down for 10ms) are a signal for the USB device software (well, firmware) to initialize device configuration. Basically to set configuration, data toggles and stalls to their defaults. (I've written USB device firmware.)