There could be several reasons:
- A dirty fuel filter
- Water in fuel
- Cracked intake fuel line
- An ignition coil that is starting to fail
- A bad diaphragm
The line that sucks up fuel in the tank is supposed to move around when the trimmer is used at different orientations. If it doesn't do this, if the fuel is low in the tank when the trimmer is used at an angle, or upside down, air can get sucked into the line and stall the engine.
On initial starting, a trimmer can splutter and stall with choke off until it warms up and reaches speed and the carburetor fills properly. Try starting and running with choke back on again or half on for several seconds before turning the choke back off. See the guide for more details.