Corbyn is a Eurosceptic and found himself leading a party that, in the main, was Pro-Europe.
Because of that he couldn't bring himself to campaign forcefully either way which left his party rudderless and with no sense of direction they could communicate to their traditional supporters.
These supporters, in very many 'labour-strongholds' voted for Brexit, a situation which has dismayed the Labour party and for which the MPs blame Corbyn and his lack of leadership.
Hence the back- and front-stabing going on....