Reason behind markovnikov's rule in hydration of alkenes via mercury catalysis
If I could postulate, the reaction is like an epoxide ring opening during acidic conditions, where the oxygen is protonated, making it electron-deficient and have a +1 formal charge. This creates a good leaving group (an electron-deficient O) and in your case, an electron-deficient Hg.

Credits: Clayden Organic Chemistry Vol 2

Since it is a good leaving group, it wants to leave and as predicted by the Sn1 mechanism, the C-Hg bond bonded to the more substituted carbon prefers to break in order to take advantage of the more stable carbocation intermediate. This makes that particular bond weaker. The nucleophile thus prefers to attack the more substituted end in order to take advantage of the weaker bond. As the bond is weaker, the net enthalpy change would be lower, making the attack at the more substituted end more thermodynamically favourable. Additionally, notice that in the transition state, the developing positive charge is more resonance stabilised if the nucleophile attacks the more substituted end, making the reaction more kinetically favourable. From both perspectives, kinetics and thermodynamics, an attack at the more substituted end is favoured.