They think he might have been a Shia -> Sunni convert ("revert").
Indeed.
About the hostage taker Man Haron Monis. Excerpt:
As recently as last week on a website he used both to defend and promote himself, he announced that he had converted from Shia to Sunni Islam. That website was shut down as Monday’s siege developed, and police asked media outlets to refrain from giving him a platform as he held 17 hostages in the Lindt cafe in Martin Place.
Sydney Shia leaders had apparently urged federal police to probe his claim to be a leading cleric, while he was ignored by the Sunni community. He had no links to the Islamic State terrorist group, and despite his criminal past was not seen as a likely exponent of the group’s ideology.
One upshot, some would argue, is that he fits bill of a classic lone wolf – a profile that had been much feared by security officials. Monis is believed to have been a self-starter, who had attached himself to the virulent worldview of Isis. His self-radicalisation appeared to be rooted in grievances against the government and fueled by his increasing marginalisation.
Earlier this month, Monis had posted on his website that Shia muslims were rejectionists – a key message of extremist Sunnis in the Middle East.
But there were other hints on the web post that Monis had become radicalised: a rambling October letter he penned to the Muslim community rejected the “new religion” of moderate Islam. ‘This pen is my gun and these words are my bullets, I fight by these weapons against oppression to promote peace,’ he wrote.
An image on the website appears to show Monis wearing the same headband that photographs suggest he donned during the Martin Place siege, reading: “We are ready to sacrifice for you, O Muhammad.”
Shortly after Tony Abbott’s government was elected in September last year, Monis sent the prime minister a letter inviting him to a live debate in which he said he would prove that “Australia and Australians will be attacked” as a result of the country’s participation in the war in Afghanistan.
More:
http://www.theguardian.com/australia-news/2014/dec/15/man-haron-monis-sydney-siege-suspect