もちろんフィードバックです： https://en.wikipedia.org/wiki/Audio_feedback この場合、トランスデューサはマイクロフォンの役割を果たす。
Every different setup has a different potential for feedback. You could get feedback with your magnetic sound hole pickup, it's just not as easy. If your acoustic has a battery inside it, then you have an active system, which means the K&K system is giving you some gain before you even hit your first pedal, whereas the sound hole pickup is totally passive and puts out a very low level.
You might have some success with a sound hole feedback reducer (the link is just one example, there are other brands available). Other ways to reduce the feedback you are getting are to lower the gain of the amp, lower the volume of the amp, lower the volume on your guitar (if it has one), stand farther away from the amp, face away from the amp, play in a larger room with more sound deadening (heavy curtains, bookshelves, carpet, acoustic treatment.