Pretty much in the context of Days of Future Present, Days of Future Past changed the future slightly, but not enough where it still converged into a similar enough future. What they didn't know was that Franklin, from the original future, had also time travelled and wound up in the new past but they did anticipate something similar and so the Sentinel Hierarchy positioned Ahab in the past to stop further time travel from changing their future. Ahab figured, well I'm going to end up killing all these guys in the future, might as well do it now, sure won't change anything against my future.
Excalibur's Alan Davis storylines retconned this into the Days of Future Past just being an alternate future (but also still contradicting details from the older stories, likely due to Claremont's influence on Davis during this time) and Ahab was going across timelines hunting for Rachel, which doesn't make much sense.