Albums get pushed back for a number of different reasons. I'll address the marketing/promotion aspect first...
The success of an album is directly related to what else is going on in the industry/world around that particular release date. Like you wouldn't drop your artists CD within a few weeks of a superstar like Jay-Z, Kanye West or 50 Cent releasing their albums...because your artists album would be ignored...
Did you know that the industry's favorite ghostwriter, Madd Skillz (the guy who does the year end Rap Up songs) released his debut album on the same day as The Score by The Fugees and All Eyez On Me by 2Pac??
EXACTLY!!
Those are 2 of the biggest selling albums in Hip Hop history, by some of it's biggest stars...nobody even knew Skillz had released an album, let alone paid attention to it!!
On the other hand, Paul Wall dropped his debut CD on like the second Tuesday in September...and that particular date had NO OTHER hip hop music of any significance being released anywhere CLOSE to his date...and he scored a #1 album on Billboard, and it shot him into stardom.
So the timing thing is based upon other artists release dates, and the season. Like Christmas is a really popular time to release an album...that's why you see superstars releasing albums around that time...as well as June...which is the beginning of the summer, and the kids are getting out of school, getting jobs, riding in their cars, etc.
But that's the quick and dirty reason as to why albums get pushed back, based on "timing".
The other reason albums get pushed back is because they either don't have a single that's catching on with the public, or the album got less than favorable results when previewed by the media/critics.
This is what happens (IMO) in most cases of release dates being pushed back. Say in the case of a Keri Hilson. She's had multiple release dates, but they couldn't get a single to catch fire. Energy didn't do it. Neither did any of the other songs that were "leaked" or released...But now that she has this Turnin Me On single burning up the airwaves, I guarantee you we're gonna see her album before May 1.
When albums get pushed back because of this reason, the labels send the artist back in the studio to record more songs. Then once the album is "freshened up", they send out the preview copies, and get feedback. If the feedback is good, then they throw out another single and hope it catches. If it does, then the album follows roughly about 2 months later. If the feedback isn't good, or the single doesn't catch on, then the cycle is repeated.
The artist has no control over release dates, marketing budgets, nothing like that...that's all the record label.
Hope this helps.