It turns out that Valve's tinkering with Steam's game discovery algorithms has had some unintended consequences, although it wasn't them who paid the price. Instead, it was the indie developers, all of whom reported alarmingly low sales.
As you'd expect, it was the developers who first noticed their revenue taking a free fall, with several reports appearing online that document the issue in detail. Grey Alien Games owner Jake Birkett's report has a detailed analysis of the sales trends pre and post Valve's tinkering.
Birkett said that the early October algorithm changes meant "Steam was only recommending some big name games instead of relevant games". Note however that the issue did not affect all indie devs equally and while some apparently benefitted from the change, others had their profits drop to near zero.
Valve now came out and confirmed their suspicions, explaining what happened every step of the way. Their Steam code changes on 05 October 2018 were intended to make wishlisted games and those that are on sale appear more frequently in search results but things went awry.
What the change actually did was "de-boosting tags in the 'More Like This' section on a game's store page", which in turn selected similar Steam products via a single tag they have in common. These were "effectively the most popular games on Steam", which Valve insist wasn't the way they meant for it to work.
The issue was further made worse by Valve's experiments on Steam's More Like This section, where they tried to improve the efficiency of the algorithm recommending the games. "This experiment ended up showing fewer products to a subset of customers, which led to a decrease in overall impressions in that area", Valve wrote.
The company insists that normal Steam traffic recovered around 19 October and has been stable since. They also mentioned that they're "constantly exploring ideas and trying new things" to provide their customers with relevant suggestions, which is likely to make traffic "go up or it may go down for any individual product."