There were indeed special circumstances in each case.
For the Expedition 46 EVA, this was an unscheduled EVA. Scott Kelly had inadvertently engaged a brake on the Crew and Equipment Translation Aid (CETA) cart during a prior EVA.
This cart moves along the ISS "railway" used by the Mobile Base Servicer (MBS) (part of the station robotics system) and the immobile cart was blocking the motion of the MBS. This rendered planned robotics operations impossible until the brake was released.
It was desirable to rectify this situation as soon as practical.
This picture shows the MBS with the CETA carts on either side of it just right of center of the truss.
I also just read Kelly's memoir Endurance, which covers this situation in some detail.
For the Expedition 55 EVA, it seems to be simply a matter of taking advantage of a quiescent period. This article states
It will be one of the shortest intervals between the arrival of a new
long-duration crew and a spacewalk in ISS Program history and is being
conducted to take advantage of a break in visiting vehicle traffic,
ahead of the launch of SpaceX’s CRS-14 Dragon in early April.
None of the EVA objectives seem particularly time-critical.