bitcoin testnet is mining about 50 blocks per minute right now - why? 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
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-&gt;GetBlockTime() &gt; pindexLast-&gt;GetBlockTime() + params.nPowTargetSpacing*2)
    return nProofOfWorkLimit;