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This question already has an answer here:

The bitcoin test network is mining about 50 blocks every minute for the last several hours. What is going on?

You can see here: https://www.blocktrail.com/tBTC

marked as duplicate by MCCCS, 0xb10c, JBaczuk, G. Maxwell, chytrik Aug 5 '18 at 3:40

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why?

Because someone set his mining software to testnet. Sorry, this answer is useless, but there is no better answer for this question :) I doubt that the owner of hash power would show his reasons. I do not want to speculate about testing new mining hardware by developers.

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There is an adjustment for the testnet when a block takes 20 minutes or longer to mine, it will adjust the difficulty down to 1. This was caused presumably because of the difference between blocktimes of 1356765 and 1356766 because block 1356767 was reset to 1.

Every 2016 blocks, the difficulty is adjusted, but it can only be increased by a factor of 4 each time.

pow.cpp as of version 0.16.2

// Special difficulty rule for testnet:
// If the new block's timestamp is more than 2* 10 minutes
// then allow mining of a min-difficulty block.
if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*2)
    return nProofOfWorkLimit;
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Bitcoin's difficulty adjusts only every 2016 blocks (though the calculation is done on the previous 2015 blocks, due to an off-by-one error), and there is a maximum limit to the degree of the adjustment. This means that when the hash rate spikes—as it does when someone points a whole rig to the testnet as @amaclin stated—it can take a while until the difficulty adjusts to the point where the block interval reaches 10 minutes again.

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