It's because the rake (angle) of the forks on a motorcycle creates larger trail and caster. This creates stability, especially on a large touring bike.
When adding two large, flat wheels out back, the bike (now trike) definitely wants to go in a straight line, and you no longer have a leaning front wheel. The steering becomes stiff and hard to turn.
This is fixed by putting a rake angle kit in place, such as EZ-Steer. This increases the rake angle, which reduces caster and trail, making the steering system less stable (which is fine, because we gained a ton of stability with the rear wheels), and easier to turn.