But in a strange move, Iran has closed the payment gateways of all exchange offices that operated in Iran and treated them like swindlers.
That's a lie because there were no "exchange offices" in Iran to be closed. There were individuals who had created a website and were buying and selling bitcoin with huge fees
without any license whatsoever. Many of them were scammers. So far some of them are shut down.
also the owners of digital currency mining machines are guilty and will be dealt with severely.
That is an exaggeration. Miners have to get a license and pay taxes in order to be allowed to put pressure on the electricity network.
What could be the reason for all this government pressure on people not to mining bitcoins?
You are exaggerating but the laws Iranian government keeps enforcing about cryptocurrencies are getting harsher. I suppose it is mainly because Iran is under sanctions and the government is trying to fill the gap left by US dollar with bitcoin. There is even a law saying bitcoin should be used for import/exports.