Microbending

Mechanical stress on a fibre that introduces local discontinuities, which results in light leaking from the core to the cladding by a process called mode coupling.
The introduction of minute curves with small radii in the optical axis of the guide or the core-cladding interface surface at which incidence angles of light rays propagating in the guide are, or tend to become, less than the critical angle so that some of the radiant power in the ray escapes from the waveguide core into the cladding and out of the guide.