Given the state of the Lions' secondary right now, it's no surprise that Seisay got promoted this week. He nearly got called up from the practice squad last week, and that was before the Lions lost Nevin Lawson to a season-ending injury. The Lions also lost Cassius Vaughn to an ankle injury for at least this week, so Seisay was needed just to give them more than three healthy cornerbacks.
With Seisay coming up to the 53-man roster, Ogletree became the odd man out. This also isn't really a surprise considering Ogletree was inactive for the first two weeks of the season. Also, since Ogletree won't have to go on waivers, the Lions have the ability to simply re-sign him on Monday if they decide to make more changes to their roster.
With Seisay promoted and Ogletree released, the Lions have 53 players on their roster and one open spot on their practice squad. Chances are that open spot on their practice squad won't be filled until next week.