standard answer is to hook up a MIDI monitor between the units (or Tee off the OT output). The OT is clearly sending stuff to the A4.
Look at the A4 MIDI spec to see what external MIDI CCs (or notes) cause the behavior you see and that should help track down what the OT is sending.
According to the OT manual, the crossfader only sends CC 30 (hex, 48 decimal), values between 0 and 127