It's the market that moves the shares. If the sellingprice on Merkur is higher on Merkur, it makes sense to buy shares on OTC then sell them on Merkur. During 2017 the share price has almost constantly been higher on Merkur, therefor tones of shares has been moved from OTC to Merkur.
What is very unusual right now is that the shareprice on merkur is still higher on Merkur compared to OTC, but the number of shares registered at Merkur is decreasing, this can't be explained by normal market forces.