Was intended to give the pardoning of Sheriff Arpaio as little coverage as possible in the mainstream media, given that hurricane Harvey was clearly the most important news to cover. Trump's message would still reach the right people among his Breitbart (white supremacist) base, because they may more attention to Trump's tweets and dog whistles.
Just to remind all of you, what Trump did in pardoning Arpaio was this:
While he is not the first official whose offense involved abuse of public powers—from Nixon on down, others fit that category—his is the first case I’m aware of where someone is pardoned for using state power toward racist ends.
Phoenix New Times provides us a list of the deeds of Sheriff Arpaio. It's not pleasant reading.
The catastrophe in Texas works nicely to reduce the publicity Arpaio's pardoning would otherwise receive.
Another possibility is that the timing of his pardon was planned to appease that Breitbart base for the simultaneous news that Sebastian Gorka, one of the white supremacist guys in the Trump administration, was let go.
Or Trump might be signaling to his co-conspirators that he will pardon them, so there's no need to make deals in the Mueller investigation etc.
Now I'm firmly in the world where hats are made of tinfoil and conspiracies sprout up everywhere.