Reading the relevant code snippets…
Especially in conjunction with the XIAO_ESP32C3/pins_arduino.h tells me, that the variant seeed_xiao_esp32c3 does not explicitly define the RX1, TX1 pins which should be used for Serial1, and thus the default macros push it to RX1 = GPIO18, TX1 = GPIO19, which however exactly are the USB D+ and D- pins.
Try to explicitly give it the pin numbers you want in the .begin() method.
Serial1.begin(115200, SERIAL_8N1, /* RX */ 1, /* TX*/ 0);