There are lots of reasons. Sometimes once you see the teams registered you decide it's either above or below your ability. Sometimes if the weather is questionable and your not local you may decide it's not worth the hotel stay for your families if there is a good chance you won't play. Sometimes players get injured or forget to tell you about family plans and you don't have a full team to bring. Life happens and while we would all appreciate as much notice as possible it just doesn't always work out.
We had to pull out of a tourney due to injury and a funeral less than 6 days prior to the tourney. With 3 out, and lack of available pick up talent, we decided it was best that we not go. It sucked but was better than going to get hammered on. We lost the entry fee. We are adding 2-3 more players for fall.
Up until the schedule is released I don't feel bad if we pull out of one. We've done this a few times, usually because we found another one that looked better. Weeks or months is not an issue at all IMO.
Pulling out of tournaments late can drive tournament directors crazy, especially after the brackets have been drawn. I think most of our bracket draws were Wed or Thur before play began on Fri or Sat.
You don't need any reason to withdraw up until the week of the tournament or the planned schedule is to be published. Once the schedule is published you better have a real reason why you are not going to play (things happen)
Now on the other side - for the team, if you were planning on a particular schedule and it changes - there better be reasons. Too tough, injuries, tournament was either way weaker or stronger than expected, replacement to a rained out weekend, etc, etc. If you change final nationals tournament it should be more of a team decision.
BTW - this is why our tournaments are all pay-up-front on registration. We used to get teams enter multiple tournaments for a weekend, wait for the schedule to come out to decide which was the better tournament and then pull out of the others. That doesn't happen anymore.
We're looking at a scenario like that for next weekend. It's a tournament 3 hours from home (hotel required). I've been looking at the list of teams registered and thinking we're going to get absolutely pounded. I guess Coach will have to make a decision by Wednesday (the day hotels become non-refundable). There's nothing quite as deflating as spending all the money on hotel, gas, and meals and going 0 for 7 or 8 games over a weekend.