This decision on 16 May is based on a security analysis that the Government invited the other Riksdag parties to take part in and its conclusions that were presented in a report on 13 May.
The decision has been taken with broad support in parliament. See link on the government’s webpage: The Government has decided today that Sweden will apply for NATO membership. - Government.se
The security analysis presented on 13 May can be found here: Deterioration of the security environment – implications for Sweden - Government.se