According to William Ladson, both pitching coach Steve McCatty and trainer Lee Kuntz noticed something was askew while Stephen Strasburg was warming up in the bullpen:

Pitching coach Steve McCatty and trainer Lee Kuntz told Mike Rizzo that something was wrong with #Starsburg. #Nats

GM Mike Rizzo just noted on MASN regarding his decision, "It's the job of the general manager to protect the assets for the club. It's unfortunate that a lot of people paid to come and see Stephen pitch, but I think it's important that Stephen's around for a long, long time."