Over the past week since version z8 was released many users have had connection and syncing issues that caused users to be unable to connect to the main blockchain or in some cases stuck mining on their own chain.
Initial reports and Github issues saw the strange peer behavior and assumed that a third party was using many peers (in a DoS attack) to prevent users from connecting with the Testnet.
The developers have spent many hours determining the cause and it is now known that it was a conflict with upstream Bitcoin code that caused the strange network issues and not a malicious 3rd party.
You can see this new issue on Github https://github.com/zcash/zcash/issues/1289 that shows the bug is related to the MAX_HEADERS_RESULTS Bitcoin parameter. It was the increase in block header size (due mainly to the size of the Equihash solution) that interacted badly with the value of this parameter from Bitcoin.
Since this issue has now been isolated it is in the process of being fixed and will be incorporated into the next Testnet release z9 which is due out sometime late this week or early next week.
(HT to @daira for the correction)