On Dec. 4, $8.9 billion in Bitcoin (BTC) were moved on the blockchain in a single hour, excluding change volume, which is Bitcoin that returns to the sender.

Bitcoin’s high transaction volume was pointed out by Rafael Schultze-Kraft, the co-founder of on-chain market intelligence firm Glassnode. In a tweet on Dec. 4, he claimed that such a high hourly transaction value was a first for the network:

“It's the highest hourly USD transaction volume in Bitcoin's history.”

Bitcoin hourly on-chain transaction volume in U.S. dollars. Source: tweet

Bittrex is responsible for the spike

Schultze-Kraft’s company apparently soon started work to uncover who or what was the reason for this unusual spike. Earlier today, on Dec. 5, Glassnode posted on Twitter claims that the spike was caused by cryptocurrency exchange Bittrex moving a large amount of Bitcoin multiple times.

According to the tweet, the exchange performed 21 on-chain transactions within one hour, each of which moved around 56,000 BTC (nearly $416 million) for a fee of about $0.60. The tweet also contained a link to data pertaining to the first of those transactions.

Interestingly, the transactions took place ahead of the exchange’s maintenance scheduled for today. Cointelegraph Spain reported on the transactions yesterday, and Bittrex told the outlet that no hack took place.

While at present it is still unclear why the transactions took place, some speculate that the reason may be that funds contained in the exchange’s cold wallets were moved.

From time to time, Bitcoin’s blockchain shows seemingly anomalous activity that is explained later. One of the latest examples took place in November, when Bitcoin’s blockchain mempool was at its highest level since January last year without a corresponding spike in the number of unconfirmed transactions.