It is not possible to test every consequence of a new/upgraded backport.
So incompatibilitys between backports and/or stable componentens may happen.
And sometimes they are even known and can't be avoided.
You are running Debian stable, because you prefer the Debian stable tree. It runs great, there is just one problem: the software is a little bit outdated compared to other distributions. This is where backports come in.
Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution. It is recommended to select single backports which fit your needs, and not to use all available backports.
I suggest changing this to something like this:
Backports are software from Debian's testing and unstable distributions, recompiled and repackaged for the current stable distribution so that they will run without new libraries whenever possible, and without upgrading to testing or unstable.
Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!
It is therefore recommended to select single backported packages that fit your needs, and not use all available backports.