Some compilers save debug information with the code - names of the variables, functions, a lot of data that is not needed with the binary but greatly helps in debugging it - and a smart debugger can generate some pseudo-C-code from it and the binary (pseudo, because it's not 100% the same as the original...)
The trick such a binary is much bigger, sometimes slower, uses up more memory etc, etc, so people generally do it only in test phase - the released binaries are cleaned of the debug symbols and can't be turned back into code. And I don't think any '51 C compiler can generate the debug symbols either.