It's just a standard treatment of a vowel plus r verb ending. Consider the following present-past participle pairs: spur - spurred bar - barred stir - stirred deter - deterred abhor - abhorred Nothing unusual going on here.
Re: occur. Surely the difference is in pronunciation. Occur isn't pronounced 'occure' 'cos it would then be a different (non-existent) word. So adding 'red' for the past tense, making the word 'occurred,' is just following the correct mode of pronunciation. I suppose 'occur' coulda been spelt 'occurr' or maybe 'okkurr' but it ain't.
Why is an additional 'r' added to occur to make it past tense? Why not 'occured'? Is there a difference in spelling between UK and US English? If not, what is the derivation of this spelling and what is the rule that drives it? Anyone have other examples of this happening?