The version I read said they changed link frequencies. Presumably they have a secondary link on a different frequency for when a transmitter is down or they need to broadcast two feeds.
In the old days you'd need someone at both ends to make the change, though, and towers are normally unmanned. Which may explain why it lasted so long with the first transmission and was short in the second...maybe the station had someone standing by at the tower to make the change when/if Max returned. I dunno. Our tower was a good 30 minute drive from the station (which is common, I think...stations are usually centrally located, but the towers are out in the boonies a bit because of the need for cheap land and freedom to stand up a heckin' big tower).
Seems the opposite, first (short without sound) was on WGN around 9:15 pm and second (longer) on WTTW after eleven