Correct spelling, explanation: the base verb occur is pronounced in a way that could imply e ending for some non-English speaking listeners ([əˈkɜːr]). This however is misleading. Since the word ends with a consonant-vowel-consonant combination, the last consonant doubles with any suffix added. Therefore occurring is the correct form, not occuring.

Definition of occurring:
verb, happening, taking place
When a phenomenon of a shooting star is occurring, some people make a wish.
These plants, occurring in the rainforest, are poisonous.

Incorrect spelling, explanation: this word is a misspelling because there should be a double r – the base word, occur, ends with CVC combination (consonant-vowel-consonant) and therefore the last consonant doubles. According to this rule occuring is a wrong form and the correct one is occurring.

Incorrect spelling

Incorrect spelling