Borland decided to develop TC++ because of the success of ZTC++. (I know some of the people involved.) Before ZTC++, C++ was a niche language, and Borland was having great success with Turbo Pascal. ZTC++ came out in 1987, and TC++ in 1990.
After the success of ZTC++ and TC++, Microsoft changed direction and decided to develop a C++ compiler, too. I heard (but was never able to confirm) that Microsoft had earlier been developing their own object oriented extensions to C called C*.
Coming from Turbo Pascal TC++ was a bit underwhelming. When I tried it didn't colour syntax, there was those weird #include and compin=ling felt so slow. So my first contact with C was pretty negative :\
After the success of ZTC++ and TC++, Microsoft changed direction and decided to develop a C++ compiler, too. I heard (but was never able to confirm) that Microsoft had earlier been developing their own object oriented extensions to C called C*.