I found water underneath wy dryer, I pulled off the vent hose and noticed water in the ventilation. What causes this and how can it be fixed

Causes

If the vent is low and the basement is could hot and cold air makes condensation.

Caulk on outside dryer flange against the house went bad and flex hose not water tight.

Outside flapper seal under its hood is bad.

Old dryer vent with no hood and water gets in.

Snow build up around vent and melts back in when it gets hot.

Dryer blowing heavy moisture from very wet cloths and not totally getting out of hose

Hope this helps

Happy Holiday

Also - if there is a bend in the hose this could make a difference. Try to have as few as possible bends in the vent hose. One is enough.