Welcome to the Lightroom Community. I'll help you figure this out. When you launch Lightroom, is a mobile device or an external drive connected to the machine? Unplug any extra storage device temporarily & check if the issue exists.
In Lightroom's preferences > General > Uncheck Show Import Dialog When A Memory Card Is Detected.
If this does not help, take the OS route. Check the AutoPlay settings of your computer.
Let me know if this helps.