How migration creates Linkage Disquilibrium
Imagine one population is fixed for A1 at locus A and for B1 at locus B. Here is a representation

--- A1 --- B1 ---


Another population is fixed for A2 at locus A and for B2 at locus B. Here is a representation

--- A2 --- B2 ---


Further imagine that the recombination rate between loci A and B is 0 $(r=0)$. Any migration rate will cause some individuals --- A1 --- B1 --- to be present in a --- A2 --- B2 --- population (and/or vice-versa). So linkage disequilibrium is complete.

If the recombination rate is in between 0 and 0.5, then there will be some level of linkage disequilibrium at equilibrium that can be calculated for a specific demography (incl. migration rate).