Much more likely that many work long hours, are in a rush to get to work in the morning and assume they will have enough time to vote after work. Most people I know (who are in their 20's so may be confused with 'students causing trouble') don't work close to their homes and won't get home until around 8pm. Or perhaps it was their first time of legal voting age and wanted to go in person!
Im sure many will use the postal vote system in future after this but many younger voters might be undecided still by the deadline to send off a postal vote and it would be fairly obvious to me that the big rush would past 6pm when everyone gets home from work so would assume that they would be prepared for this!
And to Laura Norder - Not quite sure what you mean by social networking sites unless you are suggesting that this was arranged in some 'flashmob' sort of event but Im glad that so many people of my generation are so involved with politics at the moment and are using these sites to engage in politics. Its a shame that some people are so quick to blame something like this on 'students out for fun'.
And you cant possibly blame running out of ballots on 'students'!