Usually it will be because the seat switch or the reverse switch is faulty
However it won't hurt to remove the belt from the pulley and see if it still happens.
If not then the problem is in the deck .
Husqvarna print their circuit diagram in the front of the parts book which are a free download from the Husqvarna Lawn & Garden web page .
When you examine your copy you will see there are 2 separate engine kill circuits
Seat switch + PTO switch + magneto
Reverse switch + key switch + magneto
Seat switch is obviously under the seat & easy to find which is why 99% of the brain dead morons with extra large egos on You tube & face ache tell everyone to bypass the seat switch regardless of weather it is part of the problem or not.
Reverse switch is usually tucked up under the transmission and is activated by the rod that controls the F & R motion .
The reverse switch is a simple on off plunger switch & for the purpose of diagnosis can be unplugged .
Because of the afore mentioned brain dead, the seat switch can not be unplugged as it has a shorting strip hidden within the plug .
BEcause of the shorting switch, if the plug is not tightly connected then the shorting strip activates .
More than one of my customers mowers has a zip tie around the plug to keep the halves together