Alexa ratings past the 100,000 mark are extremely susceptible to very small changes in traffic, not only on your site but also on everyone else's sites. Because it's a comparative ranking system, even if your site maintains the same amount of unique traffic, other sites may still surpass you if their traffic increases.
Essentially, sites with relatively low measured traffic will not be accurately ranked by Alexa. They do not receive enough data from their sources to make rankings beyond 100,000 statistically meaningful. (Source)
You can find more information about this phenomenon on the Alexa website.
Hope that helps! Have a super day!
Alan | Shopify Guru