SnipSign freeze due to MIDI buffer overload?

  // usbMIDI.read() needs to be called rapidly from loop().  When
  // each MIDI messages arrives, it return true.  The message must
  // be fully processed before usbMIDI.read() is called again.
  if (usbMIDI.read())
  {
    processMIDI();
  }

=> Maybe there are more than one incoming MIDI messages per loop. Try

  while (usbMIDI.read())
  {
    processMIDI();
  }