Fodder is food for animals.
In an idiomatic way, it is used in much the same way here as in the expression food for thought.
The allegations could provide Russian officials with material to create a pretext.
The word fodder usually comes with negative connotations. Although it provides something for them to create that context, it will not provide any real argument with a strong base. They can just "gobble it up" in order to create (new) propaganda.
The implication is that Russian officials will use any excuse to suit their needs, even if it does not create a very strong basis for their actions.
Another way fodder is (was?) used was in the expression cannon fodder, to describe soldiers that served as "food for the enemy cannons" - that is, their role on the battle field would consist mostly of being shot to piece by enemy fire.