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Did you read the postings I mentioned in the article?

It is explained in detail there.
Are there any questions on the details?

TL;DR:
Someone did something, fixing took some time.

I could not understand what negative vesting withdrawal is? Can one person be so strong that he can put entire blockchain on standstill?

He tried to power down a negative amount of STEEM, and there wasn't a check if the number was valid, so the whole network went down after noticing this is wrong because it didn't know how to handle this (explained rather simple)

You are right. A small bug can crash a system.