How can all events be causally related if there is no initial event?
Determinism handles the first event very carefully. From SEP's article on causal determinism:
Determinism: The world is governed by (or is under the sway of) determinism if and only if, given a specified way things are at a time t, the way things go thereafter is fixed as a matter of natural law.
That definition is written in a form which focuses on the future state of the world given a present state, sidestepping the touchy issue of first events or uncaused events.
Obviously not everyone agrees to the same definition, but this is one reliable approach with substantial acceptance.