Excess protein consumption effect on kidney
Short answer

The ammonia resultant from the protein metabolism is toxic to the body and specially kidney. (William V. et al.)

Long answer

Protein is a poly-peptide composed by amino acids. Each amino acid has an amine group (-NH2) in its composition. In the metabolism of the amino acids, this amine group is taken, and if it isn't needed is released the to blood vessels to be transported to the liver and be turned to urea.
Needless to say since the kidney filters the blood when there is excess of ammonia in the blood vessels the kidney is the first to suffer.

I've found some papers about the effect of ammonia in the kidney but none of them where about humans. (I may try to fetch some of them later.)

Why summer?
During summer water is lost through perspiration so there's less available for the excretion of urea.
I hope it helps.

References.

1.William V. McDermott, Jr., M.D. ,Metabolism and Toxicity of Ammonia