Well, it is fair to say that I don't know for certain, but: the results revolve around a very traumatic experience of being shocked.
The article even says this:
"To better understand the part these DNA breaks play in memory formation, Radulovic and her colleagues trained mice to associate a small electrical shock with a new environment, so that when the animals were once again put into that environment, they would ‘remember’ the experience and show signs of fear, such as freezing in place. "
Note the quotes around 'remember'. That's not referring to a "what did I eat yesterday" kind of memory (episodic), but something far more primitive.
The end of the article also talks about the other already known mechanisms involved in (normal) memory formation.