It depends on the context. It just popped out use less words than it was just popped out. The first one with less words is much easier to pronounce. "it just pop_dout".

Hi Linear It is possible that either one is correct, depending on the context in which it is being used. The first is much more likely to be heard and is correct if you are describing something involuntary or accidental - "it just popped out" The second is less likely to be encountered as a phrase, but could be acceptable if you are describing a process - "removing the widget from the machine was easy; it was just popped out of the holder and then examined by an expert"