I found in an other topic this answer:
When you restore Opera Mini after an hour or two (I don't remember exactly), then new tab is opened. The idea was probably to let the user search or access news faster. If there are more voices from users that they don't like it, we may change that. For now I can only suggest to get used to this or use Opera Mini more often, so that you don't exceed the time for new tab to open.
I would like to to have the option to disable this behaviour in the settings.