What is the meaning of "behind the initial outbreak" in the following sentences,
The death toll has now topped 3,000 in China, where experts say they have identified two of the virus. That team, based at Beijing’s Peking University, suggested a more aggressive strain was behind the initial outbreak and accounts for 70 per cent of cases, while a milder version is now spreading faster.
(Source : "Coronavirus ‘is like a combination of SARS and AIDS’, doctors say " By Sam Corbishley )
Does "a more aggressive strain was behind the initial outbreak" mean "a more agressive strain was spreading slower" ?