I would hazard a guess that one of two things happened. Barkeepers friend is a mild abrasive, and excessive scrubbing of the tin would remove it. If you have cleaned it with BKF several times, you may have a cumulative effect. Or the pan was overheated. Tin melts at 450 degrees. If your heat did not go over 425, the tin would not have melted. If you noticed the copper before cleaning, probably was heat, after, I would guess it was the scrubbing.
Tin is used for lining because it is inert, and nothing used in cooking would melt it other than excess heat.1