Incorrect spelling, explanation: occuring is incorrect because r should be doubled. Doubling the last consonant in a word is happening when adding a suffix to the word which ends with consonant-vowel-consonant situation. This rule points to occurring as the correct form.

Correct spelling, explanation: this form is the correct one because the base verb occur ends with a combination of consonant-vowel-consonant. In case of such combination we double the last consonant when adding any suffix; in this case -ing. This is why occurring is correct and occuring is incorrect.

Definition of occurring:
verb, being present; happening
Nocturnal animals are the ones occurring at night.
If you work in an office with a lot of paperwork occurring, you might need a lawyer to help you.

Incorrect spelling

Incorrect spelling