AFC, many materials are very dry (cedar, fir) and splinter just breathing on them. Some materials are from fast growth trees and have a helter skelter grain pattern, (growth rings), moving all over the place as opposed to an orderly straight line grain pattern.
The mill cuts timber in to take advantage the material offered, from what part of the log and how the cut is taken will produce different quality grades, (plain or quarter sawn). Cutting against the grain flow even on tight growth ring patterns can cause chipping/splintering.
Material push, there is an optimum range at how fast or slow material should be moved past the bit. Too slow = burn, too fast = splintering, stress on the router and bit and may cause a bit to snap. Each species, (hardwood/softwood) and how it's cut will have its own speed, with practice comes confidence.
As mentioned sharp bits can make or break a cut, dull bits require more pressure to move the material and may burn it and again stress out the router. Always ensure when edge profiling that you have the bit turning into the wood instead of with it. Be very careful placing the material between the bit and fence.