Sounds like good old static electricity causing the problem.
Some folk recommend all metal duct to prevent such issues since all metal duct will conduct the static to ground without sparking.
If you are using duct which is non-conductive, such as sewer pipe, or the dryer vent hose which is likely steel wire inside plastic, this does not conduct.
When the planer is not generating dust, then no static in the duct.
It is likely the planer dust has static, and this is carrying over to the plastic duct, which insulates until the spark ignites.
This is not easy to solve without changing to all metal duct.
You can try running a grounding wire, bare copper, along the inside of the duct to the dust collector, and grounding at the dust collector.
Wrapping the outside of the plastic duct with a ground wire will not solve the problem.