This has not yet been officially explained. An autopsy was held a few days after her death on 7 April 2014, but all we know from that is that the results were inconclusive - so the cause of death has not been established. Her family are now waiting for the results of some toxicology and other tests, which could take a few weeks.
The police have said that they are treating her death as not suspicious, but unexplained. This should mean that no-one is suspected of any involvement, but that they don't yet know whether she died of some undiagnosed illness, or as a result of taking something that killed her - and if the latter, whether it was an accident or deliberate.
Because Peaches' mother died of a drug overdose it's perhaps not surprising that some commentators are speculating about this possibility, but at this time speculation is all there is. It must be terribly hard for her family not knowing exactly what happened.