Physics About Physics = Metaphysics
This is a question for philosophy, since it deals in the very foundations of how physics is done.
The philosopher David Hume notes that it is impossible for us to truly know that causality exists:
Hence we may discover the reason why no philosopher, who is rational and modest, has ever pretended to assign the ultimate cause of any natural operation, or to show distinctly the action of that power, which produces any single effect in the universe. It is confessed, that the utmost effort of human reason is to reduce the principles, productive of natural phenomena, to a greater simplicity, and to resolve the many particular effects into a few general causes, by means of reasonings from analogy, experience, and observation. But as to the causes of these general causes, we should in vain attempt their discovery, nor shall we ever be able to satisfy ourselves, by any particular explication of them. These ultimate springs and principles are totally shut up from human curiosity and enquiry.
Elasticity, gravity, cohesion of parts, communication of motion by impulse: These are probably the ultimate causes and principles which we shall ever discover in nature, and we may esteem ourselves sufficiently happy, if, by accurate enquiry and reasoning, we can trace up the particular phenomena to, or near to, these general principles. The most perfect philosophy of the natural kind only staves off our ignorance a little longer, as perhaps the most perfect philosophy of the moral or metaphysical kind serves only to discover larger portions of it. Thus the observation of human blindness and weakness is the result of all philosophy, and meets us at every turn, in spite of our endeavours to elude or avoid it.
As Good as it Gets
We never know that billiard ball A striking billiard ball B "caused" B to move. We just expect it to happen because every time we test such a situation, that's how it works. Our belief in cause and effect is a prediction of the future based solely on inference from experience. This is, of course, a fragile kind of knowledge:
Let the course of things be allowed hitherto ever so regular; that alone, without some new argument or inference, proves not that, for the future, it will continue so. In vain do you pretend to have learned the nature of bodies from your past experience. Their secret nature, and consequently all their effects and influence, may change, without any change in their sensible qualities.
In the context of physics, you must simply be content with the simple answer that a magnetic/electric relationship has been found in the past. It's an explanation that works for now.