When you are mounted and run towards a herb, for example, if you right click to gather (while mounted) I'm getting dismounted and "Interrupted". The cast time for gathering starts but is then... interrupted.
This also happens when riding towards a mob and beginning to cast (while mounted). You are dismounted but then your cast is "Interrupted".
This isn't how it used to function and it's a bit of a pain :)
Edit: So, I realized that recently I'd changed to riding the Azure Water Strider. I changed to another mount and this no longer happens.
This appears to be a bug specific to the Azure Water Strider mount, which is a shame :/