Seconding what Ian posted, there are plenty of products that need to be watered in immediately after application. Timing these applications around a rain storm can be a pretty good idea playability wise, since conditions will be soggy with or without irrigation (although I can't think of too many products that benefit from 4" rain on top of them).
Another possibility is the crew was conducting a trial run to visually inspect the irrigation system, which is a good practice when a system has recently come out of winterization.