Its the equivilent of a earthquake
Neutron stars are known to have friction, and just like earth has plate techtonics (caused by friction), why should neutron stars be any different.
They slow down as the friction builds up, then it is released in a quake that speeds it up temporarilly. Once that is released it goes back to normal, and starts storing the energy for the next quake.
Surely someone has spot that, especially as neutron stars are highly compessed?