Now that we have identified that it is a kernel panic, that information may help point toward a solution. I have been reviewing some of the forum posts mentioning “kernel panic” and see a variety of possible explanations.
Since it sounds like this is not a fresh install you’re having trouble with, maybe it has to do with a dom0 update breaking boot. Before this problem started, do you recall applying any updates to dom0? If so, and if you have a backup from before the updates to which you can restore, that may fix it, as was suggested here. Alternatively, it was suggested that it could be a corrupted initramfs issue, in which case booting from a rescue disk or live system, chrooting into the system, and recreating initramfs might also fix it, which can be done with the dracut --force command.
Without further information however, it is not clear whether either address the problem. I can say that I have no reason to suspect that this has anything to do with your USB (dis)connection activities as of yet, though.
For now, I recommend you search around in this forum, on mailing lists (such as the [qubes-users] mailing list), and elsewhere online for what others have said regarding the kernel panic error you are getting, since some of the descriptions may sound familiar to your experiences. I will do so as well, when I have the time, but for now the best I can recommend are those two ideas above, that it may be a dom0 update error for which a backup restoration can fix or it may be a corrupted initramfs for which its regeneration may be the solution.
Wish I could be of more help,
John
P. S. It seems like you are not the only one with this problem, though unfortunately that thread received no further replies. So while not a solution, at least it confirms that you are not the only one experiencing this problem. This thread seems similar to your problem as well (and it has replies), so maybe it will help you.
P. P. S. This forum thread may also prove to be instructive if the problem is with a failed dom0 update breaking initramfs and resulting in a kernel panic.