Each week, FantasyPros publishes a fantasy baseball trade chart. The chart contains player values designed to help you assess the overall weight of both sides of a trade.

The weekly trade charts also keep track of the changes in a player’s value from the previous week. But sometimes, the reasons for a change aren’t obvious. “Why is player X gaining three points in value this week when player Y, who had an even better week, remained the same?”

Good question, reader. There are many answers as to why any given player moves in value from one week to the next. And in this article, we’ll examine some of the biggest risers and fallers each week, and explain the movement.

Here are some of the most notable risers and fallers in the FantasyPros Week 15 Trade Value Chart:

Risers

Corey Seager (SS – TEX). Week 15 Value: 23. Previous Value: 18. Change: +5

Corey Seager by month:

• April: .256 with four home runs and 12 RBI
• May: .214 with seven home runs and 12 RBI
• June: .222 wiith four home runs and 11 RBI
• July: .355 with seven home runs and 17 RBI

Seager has always been an all-around bat, a player who contributes in four categories while exceling in one (batting average). But that hasn’t been the case with him in his first year in Texas, at least not until the month of July.

The thing is, Seager’s xBA is .309, second-highest of his career and in the top five percent of MLB. His xWOBA is .409, which ranks in the top four percent of MLB.

Seager has been fine. Even before this recent surge. And although sometimes a player who has been traded to a new team takes a year to adjust, that really doesn’t seem to be the case here. He had mostly been unlucky. And his luck appears to have changed.

Luis Castillo (SP – CIN). Week 15 Value: 19. Previous Value: 14. Change: +5

Let’s start with the basics: in a year in which he began the season injured and was coming off a poor season, nobody wanted Castillo. And, after a rough start, he’s now pitched to a 2.77 ERA, a 1.08 WHIP, and a 25.5% strikeout rate. He has allowed one run or fewer in each of his last four starts, a span of 27 innings.

Up until May 31st, Castillo had used his four-seam fastball about 17% of the time. He had a 4.35 ERA with 18 strikeouts through four starts. In his next start (against the Red Sox), he used his fastball 49% of the time and tossed six shutout innings with 10 strikeouts.

Over his following four starts, he used his fastball 23% of the time. He had a 4.06 ERA with 21 strikeouts. In his last four starts, he has used his fastball 47% of the time. He has a 1.00 ERA with 33 strikeouts.

So, yeah, there it is. Castillo’s fastball has been dominant and he has finally leaned into it consistently. And now, we’ve reached the point where he’s highly likely to be traded within the next two weeks, increasing his win potential and bettering his home park. If Castillo continues to use his fastball roughly half the time, and especially if he gets traded to a team with a pitcher-friendly home park, he’ll continue to rise in value.

Fallers

Nick Castellanos (OF – PHI). Week 15 Value: 14. Previous Value: 18. Change: -4

Anything can happen with a veteran player. Take Seager above or Juan Soto. Sometimes players slump and everything supports their struggles and then instantly, they flip a switch.

But it’s hard to see that happening with Castellanos this year. He has eight home runs in 91 games. Eight. His ISO is .124. His hard hit rate, average exit velocity, and barrel percentage are all career lows.

Maybe Castellanos is just having one of those transition years where a player struggles to live up to his contract. Maybe he’s having difficulty after leaving the friendly confines of Great American Ball Park to the slightly less-friendly confines of Citizens Bank Park.

Castellanos is hitting the ball on the ground and going to the opposite field more than ever. Unless he drastically changes his approach, it may wind up being a lost year for the slugger. And his value will drop each week until he does.

Lance Lynn (SP – CWS). Week 15 Value: 7. Previous Value: 13. Change: -6

If, like me, you’ve waited for Lynn to come around expecting him to turn the corner at any minute, you’ve been incredibly disappointed. He now has a 7.50 ERA and 1.50 WHIP, and he’s allowed 19 earned runs over his last three starts.

Lynn’s ERA indicators are all below 5.00, so he’s obviously been a bit unlucky. But his fastball is sitting at 92.6 MPH when it has been at 94 or higher over each of the last three seasons.

His numbers will come back down a bit, of course. But he’s a low-level starter going forward, nothing more.

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Dan Harris is a featured writer for FantasyPros. For more from Dan, check out his archive or follow him on Twitter @danharris80.