Diagnosis
Debris—likely fine dust or sticky residue—has become trapped inside the trigger mechanism, preventing the spring from returning it to the off position.
How to Fix It
One: Remove the battery to ensure the vacuum is completely off. Two: Hold the vacuum over a trash can. Three: Repeatedly press and release the trigger while using a can of compressed air to blow into the gaps around the trigger button. This can dislodge the debris.
If That Didn't Work
A more thorough clean is required. This involves disassembly of the main body. As this is complex and may void warranty, contact Dyson Support for guidance or a possible repair.
Heads Up
Do not use liquid cleaners or lubricants on the trigger. This will attract more dust and can cause electrical damage. Use dry cleaning methods only.