In my world there are two dominant races, Lorani and Subloran (as seen in this question). The two have lived in a strained peace for thousands of years, primarily by ignoring each other. But in the year (earth equivalent) 1720 the Lorani enslave most of the Subloran population, capturing the few who managed to escape. This is different from earth, where the slave trade started in the 15th century, around 300 years earlier.
What events could lead this to happen at this point in tine and not hundreds of years ago? What cause would I have to place in their history(s) to lead to my desired effect?