sometimes....the person expect followup.... i promise you it is not intended to be a 1v5.
For example, i'm Wukong i see my team closeby, i'm REALLY fed, my team is literally able to followup in 2-3 seconds, maybe less if they use a gap closer.
Then it happens...a chance to 5 man ulti. so i ping really quickly and then go in. Remember, i'm really fed at this point and basically even if i die its a won teamfight simply because of all the damage i did before i died.
Instead of following up...my team walks away.
and my engage to what would have been a won teamfight had followup came, turned into what you called the winning syndrome.
Remember, if someone engages...do consider they have a reason for engaging. Don't just abandon them
P.S : In the example even though i died i still got the enemy squishy carry lol. so imagine if followup came as planned?...gee gee....