Consider a positive charge $P$ moving towards a wire that has the same density of positive charges and negative charges, but so that the positive charges are moving to the right. Then the magnetic field should cause $P$ to move to the left.
Now consider the Lorentz transformation that makes $P$ still. Now the wire is moving to approach $P$. But because $P$ is still, it cannot be affected by any magnetic effects, so any force on it is caused by electric effects.
So now we have a wire with positive charges moving to the right approaching $P$, and somehow this creates an electric force on $P$ to move to the left. How?