Can confirm that having wifi turned on is causing the problem with me old S2 watch.
I had wifi turned off for almost 3 weeks and each night on the charger, no problem with charging. No random reboots, no chimes, nothing.
Last night i decided to turn back on wifi and soon as i did and placed my S2 on the charger within a few mins it started to chime and show the charging animation every 5 mins or so and then it began to reboot with the Apple logo.
Turned off wifi, put it back on the charger and its fine again.
I dont understand how this issue has happened since i haven't updated the OS on my S2 in a long time because there isn't a newer version available.......really really strange.