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Jealousy. He basically dismisses it by pointing out that while some of the Brothers have resisted the narrator's ideas initially, once the idea itself is worked out, they have been in favor of the results. He says the letter is most likely from someone who resents the narrator coming in and doing things they should have done a long time ago. Also, they don't want the narrator to gain too much power too quickly.
To what does Brother Tarp attribute the warning letter?