Good question, that probably should be addressed in TFA, but isn't. Have a cookie, assuming you're not blocking them. :)
The Earth was exiting a period of relative geological and climatic stability and entering a cooling phase, which would have helped strengthen the AMOC. This process was then enhanced by a large scale volcanic eruption, thought to be in North America, with the ejecta from that and a series of subsequent eruptions leading to a significant deviation from the trendline, a mini-iceage known as the Late Antique Little Ice Age (LILIA) similar to the Maunder Minimum, a multi-decade period of cooler than statistically expected temperatures (up to 2.7C cooler than average in European summers). This is reflected in tree-ring records which show highly stunted growth for the time, ice cores from polar ice cores, and some of the remaining writings from the period that describe widespread crop failures.