I think it happens when at closing auction someone places a...

I think it happens when at closing auction someone places a large bid or ask at the last second that would shift the SP too much. To give the existing auction participants a fair go, the closing auction is extended for 5 minutes so everyone can see the large additional bid/ask and make decisions based on it.

i think its to prevent people from manipulating the price by waiting till the last 30 seconds of auction to dump shares etc.

I'm just guessing at it from what I've read about it but I'm not really sure. Happy for someone to correct me