This is a constant and extremely annoying problem for me too on my Series 3 cellular capable watch since upgrading to 6.0.1. This problem really makes the watch unreliable, especially if you're using the watch's onboard timer. Almost burned up my steaks last night because the watch kept locking itself while on my wrist. The timer alert won't sound if the watch is locked after the timer is started.
I'm using the same watch band, nothing has changed about my wrist, the band has not been loosened or tightened (it's a stainless band that adjusts only when taking out links in the band, which I haven't done), and the hardware is functioning fine, otherwise. The only change is the upgrade to 6.0.1.
There are a myriad of other bugs in 6.0.1 that really negatively impact the user experience with this OS version (watch is often slow to respond taking a few taps to get the screen to illuminate, Siri performance and accuracy has been noticeably compromised, the reminders app on the watch has been turned into a syncing nightmare, including Siri saying she wan't able to create the reminder and she actually does, to name a few biggies).
Eagerly looking forward to a substantial update. If you haven't updated to Watch OS 6 yet, better wait for the next update of it. Unfortunately, most of the other upgrades that are a part of 6.0.1 start to lose their luster very quickly with the core, backbone functioning of the watch being so negatively impacted, as is the current case with 6.0.1.