The device increased intensity on its own.
If the increase was noted toward the latter half of the therapy session, the habituation ramp is too steep. The device is designed to increase intensity throughout a therapy session to compensate for nerve habituation. For some people, this ramp is too steep, in which case the therapeutic intensity should be decreased during the therapy session. If you allow therapy to run for 30 minutes, the device will remember the adjustment for future therapy sessions.