I think fats can stimulate the bowels to stronger contractions. I think this might be different for everyone -some more so, some less so. For bad flare ups usually a bland diet with not much fat is recommended, for instance eating dry toast instead of buttered etc.
But we all need some fats, so maybe there are ones that you might find out don't suit you, and others that do? I am usually OK with butter in moderation and OK with egg yolks, and even one kind of milk chocolate! but margarine and some oils do me in.
Heated fats might be harder to digest, like anything cooked in hot oil. I can't have fried eggs nowadays but always boiled instead.
But it's true that it's usually possible to get away with eating certain things when in a constipation or more normal state (If you have IBS-D.) but not possible when going into a "D" flare up. I almost welcome my little glitches of constipation because it does mean I can eat something different for a change/
There are also other causes for not digesting fats well. See the doc. about it maybe?