"Crooked Hillary" and the people supposedly protecting her were part of a recurring theme for Trump in the days leading up to that tweet, driven by what he watched on Hannity, etc. Here is a selection of tweets: On 7/22 1:44, "So many people are asking why isn't the A.G. or Special Council looking at the many Hillary Clinton or Comey crimes. 33,000 e-mails deleted?" Then on 7/24 2:49, the same thing, except now calling Sessions "beleaguered." On 7/25 12:03,  "Ukrainian efforts to sabotage Trump campaign - 'quietly working to boost Clinton.' So where is the investigation A.G. @seanhannity." Same night at 12:12, "Attorney General Jeff Sessions has taken a VERY weak position on Hillary Clinton crimes (where are E-mails & DNC server) & Intel leakers!" And finally at 12:21 on 7/25, "Problem is that the acting head of the FBI & the person in charge of the Hillary investigation, Andrew McCabe, got $700,000 from H for wife!" This did not come out of the blue. I'd say timing is coincidental.