Why is Little Lin's nose bleeding? 还不习惯北方的气候，估计是天气太干。
The weather caused the problem of his nose. Specifically, he suspected it's too dry. 

It seems that you misunderstand the word 估计 in the context. It's not an estimation actually. It's his suspection instead. I'd interpret 估计 as reckon, suspect, feel, and etc in this context. 

So, 


  我还不习惯北方的气候，估计是天气太干 // I am still not accustomed to the north's climate, I reckon (it's because) the weather is too dry.