Without knowing the particulars of how your code is written, here is my best guess at what is going on...
Unity likes to be helpful and will try to resolve a dependency chain if it can, even if one of the types in that chain hasn't already been registered within the container. What this means is that if class A depends on class B and you have only registered class A within the container, Unity will attempt to construct a new instance using reflection to see if it has enough information to do so. It will look for a default/parameter-less constructor or any constructor whose parameter types are already registered within the container. Failing that, it attempts the same process with each of the parameters of the class B constructors until it either builds up a full dependency tree or fails.
There are 2 potential solutions that I can think of really quickly:
1) Register the RadDatePicker
in the Unity container and specify how it should resolve the constructor parameters. See the Unity documentation for how you register a type that doesn't have a default/parameter-less constructor for more information.
2) Fork the code for Unity.WebForms
) and edit line 92 of the UnityHttpModule.cs
class the include a check for the 'Telerik
' namespace in addition to the 'System' namespace. This will prevent Unity.WebForms
from attempting to dependency inject anything from Telerik (unless you explicitly register it with the container).
Hope that helps!