My experience is similar to g1smd: In most cases when you have many redirects, it is owing to introducing new URL structure across the site.
What I saw is that in these cases Google spiders new URLs with a great speed - it is almost as it gives a priority to URLs that it has never seen to re-spidering existing URLs. The result is that for a while you may have two URLs in index: the new one and the old one that redirects, but the URL that redirects has not been spidered yet.
I have also seen old and new URLs swapping in ranking for some weeks (despite redirect) and then eventually settling on new URLs.
I am just about to put live URL restructure on a site that has 1500 pages, but where Google has indexed 8000 URLs + has further 16,000 as "Not selected" (owing to classic mistakes in parameter capitalisation, parameter order, extra parameters, dates in URL and so on). So we will be introducing 1500 new URLs, redirecting aproximately 1700 URLs and leaving other 22,000 go 404. The site itself is only 18 months old so it will be interesting to watch what happens, how long will fall last and how long to recover.