here is an article describing it
the thing is, if those taxes get cut, that will mean the trust fund for those programs will be depleted sooner than expect. those benefits are already on a path to eventually get cut within the next ten to fifteen years, and if he cuts that funding, the benefits will be on a path to get cut sooner. this includes benefits for those who are disabled. the government made a promise to give benefits back for what has been paid, but if trump does this, the government will sooner than expect renege on that promise.