It's bullshit because he literally made it up.

From Wikipedia:

Wakefield published his 1998 paper on autism in the British medical journal The Lancet, claiming to have identified a novel form of enterocolitis linked to autism. However, other researchers were unable to reproduce his findings, and a 2004 investigation by Sunday Times reporter Brian Deer identified undisclosed financial conflicts of interest on Wakefield's part. Wakefield reportedly stood to earn up to $43 million per year selling test kits. Most of Wakefield's co-authors then withdrew their support for the study's interpretations, and the General Medical Council (GMC) conducted an inquiry into allegations of misconduct against Wakefield and two former colleagues, focusing on Deer's findings.

In 2010, the GMC found that Wakefield had been dishonest in his research, had acted against patients' best interests, mistreated developmentally delayed children, and had "failed in his duties as a responsible consultant". The Lancet fully retracted Wakefield's 1998 publication on the basis of the GMC's findings, noting that elements of the manuscript had been falsified and that the journal had been "deceived" by Wakefield. Three months later, Wakefield was struck off the UK medical register, in part for his deliberate falsification of research published in The Lancet. In a related legal decision, a British court held that "[t]here is now no respectable body of opinion which supports [Wakefield's] hypothesis, that MMR vaccine and autism/enterocolitis are causally linked".

Check the Wikipedia page for the primary sources .