Usually esophoria is associated with an uncorrected hypermetropia, caused by excessive stimulations of the inner straight eye muscles that cause an excess of accomodation. In hypermetropia, to observe at a certain distance, the convergence should be less than the accomodation but because they are connected to each other, one finds that to observe an object at a distance of one meter the convergence will be a metric angle. With an uncorrected hypermetropia of 1.00 D, the accomodation effort has to be 2.00 D to see sharp at that distance; therefore either the system adapts the convergence with the accomodation or also the convergence must be of 2 metric angles. In this way however one should have dyplopia and therefore the psychomotoric system will try to balance by either compromising blurring with a doubled image or suppressing the image of one eye.
Certainly the muscular inconvenience can be severe and the pain may be hard to stand.
With the use of lenses that correct the hypermetropiatheis unbalance is virtually eliminated and there will be a tendency to return to the original conditions. This process may be accellerated using lenses too powerful for the ametropia which, inhibiting more the accomodation than the convergence, invert the process that started the esophoria.
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).
Related article: How to correct hypermetropia (3/3)