In 2004, Thomas Jones and Tiki Barber started the year as 2 of the top 4 RBs. Tiki was No 1 for 4 weeks, while Thomas Jones was in 4th place for the first four weeks.
Thomas Jones ended the year barely cracking the top 20, at 19th and Tiki dropped out of his 1st place spot down to 6th. So what happened?
In Chicago, Thomas Jones suffered from the loss of the only viable QB on the Bears team. After Rex Grossman went down, Thomas Jones went down with him. Similarly, when Eli Manning took over as QB in New York, Tiki's numbers suffered as well. His TDs and yards declined as defenses wanted to force Manning to pass. He later resurrected his production as Manning began producing.
When drafting a RB, you have to make sure that you draft a RB on a team with offensive stability. This means, no QB controversy, no RB controversy, and no posible change-ups at QB. If your RB is on a team with a lot of question marks at the QB position, you may want to trade your RB for a more stable back.
I had Thomas Jones this year as he was a top 3 RB. When Grossman went down, I attempted to trade him away for Curtis Martin, I told the owner of Martin, "Now that Grossman is down, they will have to rely on Jones more." I knew I was lying, but he didn't want to make the trade.
The guy who had Tiki in one of my leagues also tried to trade him when Eli was given the starting role. He almost got Domanick Davis for him. Eventually, Eli Manning caught on, and Tiki became a formidable back all over again. He eventually finished as the top back in most league scoring systems.
The conclusion is, a steady QB provides for steady RB production. Never draft a RB on a questionable offense, nor one on a team with a possible QB controversy. Never forget this lesson.