Due to the subway strike, the city will run extra bus lines.
Because of the subway strike, the city will run extra bus lines.
Due to the subway strike, the city will run extra bus lines.
Because of the subway strike, the city will run extra bus lines.
The preposition "because of" can state a cause that modifies the action of a clause. The word "due" is an adjective, so "due to" must always modify a noun; here, "the city" itself is not "due to subway strikes."