In fairness this doesn't happen so much on the Underground, but my local SouthEastern station has ticket barriers with Oyster as well...
1) Being stuck behind a group of people who don't get their tickets or oyster cards ready and fart around for 5 minutes looking in various bags/pockets etc. Especially when they could have been looking while queuing behind someone else who has just done the same thing.
2) Being stuck, in the rain, queuing to get off the platform partly because of the above people, and partly because people are scared of being wet even for a couple of seconds, so put down their umbrella at the last second before they go and do the above.
If that doesn't make sense, you probably need to know that my local station has the ticket machines outside, but the station roof has been extended to cover them.