A restriction in the vent stack will cause what you describe
Exactly the revent is supposed to keep this from happening. The
plumbing also might not have been installed correctly in the first
place. Another possibility is a restriction below the level of your
toilet that backs up only when there's a large load. The suds in the
water make it seem like a washing machine. If this has been happening
since you moved in you could have a case against the condo developer or
whoever sold the condo to you. Sewer gas escaping into your bathroom
would be a health hazard.
Richard