Punching through
If you make a portal, you basically punch through some part of reality. A portal is open at two sides, so you're required to punch through twice.
If you open another portal it'll need an exit. The portal will automatically probe for weak points where it can establish the other portal. Nothing is weaker than an already punched hole, so a connection is easily made with preexisting holes. That's why they made two at the same time at first so you know where to expect the holes. If a parallel universe punches a hole, it has a good chance to find the existing portals of normal Earth. If it must be random, the energy used 5o punch holes might inadvertently weakened parts of the parallel universe, punching through on the fourth hole. This can conceivably be moticed by the parallel universe, so they might have been ready.
If you have several holes, you can go through and end up at any of them. If you calculate how to end up at hole x from hole y, your calculations will only be correct if you assume the right amount of holes. That's why they sometimes end up elsewhere from where they thought they would be.
