It's nothing new, but it has gained steam in the last decade, and it isn't slowing down any time soon.
It's perfectly fine to respond with "who?" Grayson is a product of LSU, but never played for Les Miles or Ed Orgeron. Instead, he was busy melting track spikes, earning All-American honors seven times between indoor and outdoor seasons in the 200-, 400- and 800-meter events.
Grayson won four national titles as part of LSU relay teams, and turned his top-flight speed into an impressive workout and shot at pro football. The prospect was clocked at 4.33 seconds in the 40-yard dash, spanned 10 feet, 7 inches in the broad jump and posted a 34.5-inch vertical leap at LSU's pro day, according to the school's athletics website.
Grayson last played organized football in 2011 as a senior at Rummel High School (Metairie, Louisiana). Seattle brought the speedster in for a visit after his workout at LSU's pro day and signed Grayson, which they were able to do because he was a fifth-year senior who didn't play football.
This is far from the first track star to make the switch to the NFL. "Bullet" Bob Hayes was an Olympic gold-medalist sprinter who grew to become an excellent wide receiver and one of the game's first modern deep threats, as well as an effective punt returner. Dallas doubled down on track stars in that 1964 draft, also selecting Oregon track and football star Mel Renfro in the second round (Hayes was chosen in the seventh round). Both are in the Pro Football Hall of Fame.
We've seen plenty of these examples go boom and bust since then, with more conversions coming from basketball to football (I.e., Antonio Gates, Jimmy Graham), but there have been track stars in various events also doubling as professional football players. Niners receiver Marquise Goodwinwas a participant in the U.S.A. Olympic Trials in 2016, as was former USC corner and current draft prospect Adoree' Jackson.
It's a signing that carries unknown potential, which is still better than average potential. Knowing Seahawks GM John Schneider's knack for discovering talent overlooked by the rest of the league (see: Doug Baldwin), we wouldn't be all that surprised if this worked out in Seattle's favor, too.