Correct spelling, explanation: occurred is a past participle of a verb occur. Past participles are created by adding a suffix -ed to the verbs, although there are some other rules. When there is a monosyllabic verb ending with a vowel and a consonant, the last consonant has to be doubled. Hence, the past participle od occur is occurred. That makes the form occured incorrect because there is one consonant r missing.

Definition of occurred:
verb: past participle form of occur
– to take place or happen (usually unexpectedly);
The incident occurred shortly after she left the office.
– to exist or be present;
Violence among children occured at every step of education.
– to come to mind (usually followed by to);
The thought occurred to me while I was reading about these horrific events.

Incorrect spelling, explanation: occured is an incorrect version of a past participle from the verb occur. The general rule to create past participles is to add the suffix -ed to a particular verb. Although there are a few more detailed principles. If there is a monosyllabic verb ending with a vowel and consonant, the consonant needs to be doubled. That is why the correct form is occurred and not occured.

Incorrect spelling

Incorrect spelling