If and why is it bad to scrypt a bcrypted password?
The combined strength of two key stretching algorithms is not greater than the sum of its parts. It is at best equal to the sum of its parts.

If you budget $x$ amount of CPU time to password stretching then you have to decide, do you give 100% of $x$ to scrypt? 100% to bcrypt? 50-50?

You can't give both 100% of $x$ to both scrypt and bcrypt because that's the same as doing a 50-50 split with double your initial time budget.

There is one important detail to scrypt that makes it better than bcrypt and PBKDF2. And that is that it is designed to use a lot of RAM. Storage with random access is hard to implement on ASICs in such a way that gives password crackers a significant advantage of the password-defender. The RAM use makes it hard to speed up cracking with GPUs as well.

Scrypt has a cost parameter. If you decrease it then it decreases both the amount of time that it uses and the amount of memory it uses. Bcrypt can be cracked faster than scrypt when scrypt is allowed high memory usage (and an equal amount of password-defender CPU time). So, when you take time (and thus memory) away from scrypt to give to bcrypt, you're letting a password cracker perform more guesses using less of their own resources.