First of all, it wasn't exactly $7100 but $6200 over Bitstamp by a single whale claimed by a trader who said he already had such information of this incoming dump by his internal sources.

Source: https://dailyhodl.com/2019/05/17/huge-whale-sparks-major-bitcoin-btc-crash-says-crypto-trader-plus-ripple-and-xrp-ethereum-stellar-tron-cardano/

A pump cannot be carried by a single whale, but a dump can.

Reasons? See this:

- There were a lot of Shorts even when $4500 was on the table as bears were still looking to take control back from the bulls, but the exchanges were suffering losses which made it them want and escalate this unusual bull run for obvious reasons of their financial benefits. It was shot above $8000 in just a few days which made shorts lose their positions as well as a lot of money in those leveraged trades.

- This created an environment where it all looked surprising to everyone as how did the bull run start so quickly and all of these traders started following the trend which made them "BELIEVE" that BTC has now turned completely bullish and it can't turn back. Then comes the whale (I probably believe he's a bearish minded trader currently or tried to recover his losses through selling a massive amount of BTC in a single shot to drag down BTC to $6200 at a single exchange). According to the article, this made other traders try to figure out how to buy BTC over Bitstamp and make the best of this advantageous opportunity by doing arbitrage over other exchanges. So, to stop them from benefitting through this, other traders over different exchanges followed Bitstamp pattern and threw the price to $7k which could have made really handsome bucks to that whale if (and he probably have had) he had leverage trades open with high leverage towards down. Else, just give me one reason as to why someone would just dump an asset over a single exchange when he could have sold them in parts over different exchanges?