Hello, nerval.

Thank you for contacting us.

It is indeed our app BookMacster or Synkmark which launches Opera momentarily to copy bookmark changes into Opera.

For other browsers, if you have BookMacster, you can adjust a setting setting to not launch browsers to coordinate syncing.  Unfortunately, a year or so ago we removed this feature for Opera because a change in Opera's design made it impossible for us to export changes to Opera while Opera was not running.

To reduce the annoyance, the alternatives at this time are to either (1) switch of the "Export" checkbox for Opera in the browser/clients list, and export to Opera manually when needed or (2) make Opera a Login Item and leave it running in the background all of the time.  Alternative (1) makes sense if you do not use Opera very much, and alternative (2) makes sense if you use Opera regularly.

In a future version, we would like to maybe give you a preference setting to "hold changes until browser is launched".  There might be some issues with that, the most notable is that if you have Opera on a second device (iOS or another Mac) and are using Opera Sync for bookmarks, your changes will not reach the other device until you launch Opera and let our app do its syncing on the first device.  But, any case, the work to add this preference is not yet scheduled.