This has been happening to my iPhone 6 as well. I can find no rhyme or reason to it. I don't listen to music at all on my phone. I tend to go for long stretches where my lock screen is normal, and shorter periods where this RW/pause/FF keeps returning.
Several times, I've tried to determine if it was being caused by an app, so I closed my apps one by one while checking my lock screen. The first time, it only disappeared once I'd finally closed all my apps (safari was the last one I closed). Another time, it disappeared once I closed my mail, but I still had safari and one or two other apps open.
It is very annoying and it would be a great relief to know what is going on and how to stop it.