TL;DR: The aim is to discourage reorganizations and increase privacy.
In wallet.cpp, just above where nLockTime gets set, it says:
// Discourage fee sniping.
// For a large miner the value of the transactions in the best block and
// the mempool can exceed the cost of deliberately attempting to mine two
// blocks to orphan the current best block. By setting nLockTime such that
// only the next block can include the transaction, we discourage this
// practice as the height restricted and limited blocksize gives miners
// considering fee sniping fewer options for pulling off this attack.
// A simple way to think about this is from the wallet's point of view we
// always want the blockchain to move forward. By setting nLockTime this
// way we're basically making the statement that we only want this
// transaction to appear in the next block; we don't want to potentially
// encourage reorgs by allowing transactions to appear at lower heights
// than the next block in forks of the best chain.
// Of course, the subsidy is high enough, and transaction volume low
// enough, that fee sniping isn't a problem yet, but by implementing a fix
// now we ensure code won't be written that makes assumptions about
// nLockTime that preclude a fix later.
txNew.nLockTime = chainActive.Height();
// Secondly occasionally randomly pick a nLockTime even further back, so
// that transactions that are delayed after signing for whatever reason,
// e.g. high-latency mix networks and some CoinJoin implementations, have
// better privacy.
if (GetRandInt(10) == 0)
txNew.nLockTime = std::max(0, (int)txNew.nLockTime - GetRandInt(100));
As Martin Habovštiak noticed, the sequence number is set to
MAX-1 for every input. It's in the code just below the previously mentioned code section.