The only way to turn off the satellite transmitter/modem from the flight deck is to drop the left AC bus.
Agreed. However, deliberately shutting down half the aircraft's AC electrics, even for a short time, would be a major step to take, with serious repercussions. I'd be very surprised if a pilot would do that just to interrupt the satcom system. There are far easier ways of disabling the ACARS & satcom if that was the intent.
The satellite log-on at 1825 may have been caused by a power interruption on the aircraft, or it may have been caused by something else. In my experience, aircraft occasionally 'lose' the satcom for a short time time in-flight and subsequently log-on again automatically.
If the log-on was caused by a power interruption, then possible causes include a malfunction in the L Main AC Bus, causing the system to isolate itself automatically; or deliberate actions taken by the flight crew to achieve the same result. That's not something the flight crew would normally contemplate, except perhaps in the case of an electrical fire where they might attempt to isolate part of the system. As I said above, I think it is highly unlikely they would isolate half the aircraft's AC electrics just to turn off the satcom.