Why light radiates out of the bend in dielectric waveguides? 
Why EM wave radiates out of the waveguide. Is this similar to centrifugal force?
 A: Because the conditions for the total internal reflection are not satisfied: the waves are incident at the waveguide border at a too sharp angle.
A: The confinement of wlectric field intensity within a dielectric waveguide depends upon refractive index, wavelength, and geometry of waveguide.
The total internal reflection is mostly used in ray model to diescribe the acceptance angles and propogation of light in optical fiber. The bending of optical fiber changes the incident angle of light making it smaller than critical angle.

For electromagnetic radiating outside of waveguide, it's better to take wavelength and refractive index in consideration.
If wavelength is higher than waveguide geometry, higher modes will leak out.
If index contrast of waveguide is low, radiation will leak out easily for higher wavelength.
There are still some waveguide with really small radius and still don't radiate the light because of high-index contrast.e.g.
Resonators are made up of SOI waveguides having silicon core (n=3.47) surrounded by a silicon oxide bottom cladding (n=1.44) leads to very strong confinement allowing the light guiding in bends with very small radii without radiation losses.

