During contract negotiations, if an actor refuses to take a pay cut on his or her per-episode salary, the producers will often cut their "guarantee"--that is, the guaranteed number of episodes in which the actor will appear, which accomplishes the same goal of saving money for the show. The show tries not to exceed an actor's guarantee because they have to pay the actor extra for the extra appearances. The show might have cut his guarantee once they realized he was not going to be front-burner for the next few months, or he might have simply asked for more time off while other stories are playing out.
Longer-term actors (or those with better relationships with the producers) might get more leeway in WHEN those appearances happen--that is, they might be able to negotiate a longer vacation and have the show write around the actor's unavailability. Of course that will hinge on whether their character is needed. In Don D's case, it might be a good chance for him to take a longer period away from the show as the show is focused so much on Sheila and Deacon right now. If, for example he was guaranteed seven episodes per month (7x12=84 episodes per year) but took two months off for a vacation or another project, then his 84 episodes would occur over a ten-month period, putting him onscreen more often than he otherwise would have been (about 8.5 episodes per month versus seven). A guarantee works both ways--he gets paid for those episodes whether he appears in them or not--so it's in the show's best interest to get their money's worth by having him (or any other actor) appearing in those episodes.
Several years ago on Y&R they had a situation where an actress (Melody Thomas Scott) had already fulfilled her guarantee well before the end of her contract, and the show's budget was being run so strictly that they had to abruptly write her character out of some major, front-burner stories. Well, they didn't HAVE to; they could have just continued to write her into episodes and just paid the actress for those extra appearances. But money was apparently so tight that they sacrificed quality of storylines to the Almighty Budget. MTS sat out a few months (IIRC) and waited for her next contract cycle to begin before the show was willing/able to write her character back in...all while the fans were up in arms over her sudden disappearance. So these per-episode guarantees are VERY important to the bean-counters behind the scenes--sometimes more important than story considerations.