VPS providers only support a set of kernels. Yum updates can sometimes remove old kernels, which can cause problems if the VPS provider doesn’t support the newer kernels yet. Yum supports pinning a certain kernel, so you can be sure that the system can boot after an upgrade and have an orderly kernel upgrade when the time comes.

From this article:

# prevent yum from deleting old kernel
yumdb set installonly keep kernel-core-3.17.4-301.fc21.x86_64

# allow yum to delete old kernel
yumdb del installonly kernel-core-3.17.4-301.fc21.x86_64