That's not strictly true; rather, it's that all the players in contention won (except for Wesley So, who was playing another contender).
Anish Giri entered the round half a point ahead of Magnus Carlsen and Shakhriyar Mamedyarov, a point ahead of Vladimir Kramnik, and a point and a half ahead of So and Viswanathan Anand. Giri had the white pieces, but was unable to achieve anything against Sergey Karjakin, and the game finished in a speedy draw. Everyone else (except for So) took advantage.
Let's start with the big dog: Carlsen, against So. Despite playing with White he got nothing out of the opening and was maybe a little worse. But So, one of his regular patrons, played too submissively (18...Nd4 was a move repeatedly noted by Carlsen as an example of this unfortunate tendency), and Carlsen escaped to a better ending a pawn up with rooks and opposite-colored bishops. That should have been a draw, but So didn't play it as well as he could have. Still, Carlsen decided to transform it into another ending - which again should have been drawn with best play, but where best play wasn't at all easy to achieve. Carlsen gave up his bishop for a couple more pawns, and So was unable to solve the problems of that new ending. It wasn't a masterpiece by Carlsen, but it was a great illustration of why he's the #1 player: his mental strength and his ability to keep posing new problems, hour after hour, and to take advantage when even the strongest opponents slip, far exceed his competitors' abilities in those respects.
Case in point: Kramnik vs. Maxim Matlakov. Kramnik won and posed lots of interesting problems for Matlakov, but time after time Kramnik would meet his opponent's error with one of his own. Kramnik is an all-time great, and he's not doing badly here, either, but his current form isn't going to win the Candidates, never mind a world championship match against Carlsen. For his sake, hopefully it's just a matter of rust, and he'll be fully ready in March.
Kramnik is half a point behind the leading triumvirate, so let's return to the leading triumvirate. We haven't mentioned Mamedyarov's game yet, a 21-move bludgeoning of Peter Svidler. Svidler had White and played the unusual 6.Bf4 in the Ragozin. That wasn't a problem by itself; in fact, Svidler defeated Giri with it in 2015. But after 6...Ne4 his 7th move was a strange novelty that probably wasn't prepared beforehand. (What he meant to do, or what he was getting mixed up, isn't clear.) After this Black had the initiative, but it wasn't out of control until 11.Bg2(?). After this Black was better, and after 15.Qb3? (I suspect Svidler would add the second question mark) 15...Na5 followed by ...Nc4 the game was just over. Mamedyarov played well, but Svidler was unrecognizable.
Finally, Gawain Jones's tournament is starting to crumble a bit. After losing a won position against Carlsen in round 8 and failing to convert a won position against Hou Yifan in round 9 (though he was also lost at one point against her as well), he ran into some excellent preparation against Anand in this round, round 10. I'm not sure if Jones really was prepared for Anand's idea, but if he was he mixed something up and was lost almost right away. Anand won convincingly with the black pieces, and although he's a point behind the leaders he's playing well and will have two white games of the remaining three.
Tomorrow (Thursday) is the second and last rest day of the event (they played in Groningen today; it's back to Wijk for the remaining games). Today's games, with my notes to all the aforementioned games but the very long adventure story that was Carlsen-So, are here. (The other two games were Wei Yi-Caruana, which was a short draw; and Hou Yifan-Adhiban, which was a very long draw.) And here are the pairings for round 11, on Friday, featuring above all a clash between two of the leaders, Mamedyarov vs. Carlsen: