Well, the point of the detergent is to denature the protein, so I'm not actually sure but my guess would be the membrane wasn't blocked properly, so if you haven't used antibodies yet, I would do it again with BSA in TBS-Tween (Triton-X 100 can be used instead of Tween, in either case with a concentration of around 0,1%) Non-fat milk in TBS-Tween works well too.

I thought the same. I re-blocked with 5% BSA in 0.05% TBST, washed with TBST to remove unbound moeties especially ones that were left from BSA in water,  then incubated it at 4 degrees with BSA in TBST overnight to make sure it blocks fully and properly. Thank you for your answer 

I don't think the detergent is used to denature the protein (BSA). Tween and Triton are not generally considered as denaturing detergents. I think they actually are themselves blocking agents, as well as the BSA. (If the BSA were denatured, it would aggregate and precipitate.) The detergent is also useful as a wetting agent. The TBS is a buffer at physiological pH and ionic strength that is helpful for maintaining the stability of antibodies and detection enzymes.

Blocking with BSA in distilled water instead of TBS-Tween would probably still work, but might not be as efficient or complete. It would not be a good idea to leave out the TBS during the primary or secondary antibody incubations because the pH would not be controlled, and the low ionic strength might favor non-specific antibody binding.