Looks like they were waiting for media reports of the Florida attack before mailing the letters to the Senators. The letters to Leahy & Daschle were mailed as soon as practical after the 5 October news reports of Stevens' death.
Whosoever did them, the anthrax mailings were a two-phase covert op. Phase One: AMI, also New York Post, NBC, CBS, ABC. Phase Two: Leahy and Daschle.
Phase One in a 2-stage op is a set-up phase, and AMI was crucial in that set-up. It put people in a certain frame-of-mind desired by the mailers.
The mailers waited for that frame-of-mind to be established before launching Phase Two. Their aim: the weaponised anthrax of Phase Two was to be interpreted in the context construited by Phase One.
*Some possible motives: Punishment of the administration for refusal to sign a BW limitation agreement;destabilization of the administration for political reasons,etc.,etc.
Is this just a general comment or are you thinking of something specific? What frame of mind do you mean?