presuming that yours is a fully pumped system and not gravity hot water i would be suspecting that your system could be sucking in air, the air takes up volume within the pipework thus forcing the water up the open vent.
this is only a guess as ive only been a trainee heating engineer for 2 years and no-one fits the old style systems anymore and i'm a gas engineer rather than a plumber.
from what ive learned so far the main reason for sucking in air is something to do with the relationship between the pump and the open-vent.
the only other reason i know of for the open vent discharging water is poor circulation of water thru the boiler so the water is over heating and thus expanding up the open vent, this could be due to a faulty pump, thermostat or blocked heat exchanger.
perhaps a non-return valve could be fitted to the washing machine supply pipework too.
theres quite a few guys on here who know a lot about plumbing and should give you a better answer than me, gasman will probably have a good answer for you too.