If your wedding ceremony and reception venue are in separate locations, there is usually a gap of time between the end of your ceremony and the start of your cocktail hour or reception. While you are off getting your pictures taken, your guests are on their own. If they are from out of town, or have come from a distance, what are they to do between the two events? Most likely they will head straight to the reception venue because they have nothing else to do.
This might work well for the guests, but your reception venue and other Wedding Professionals might not be ready for that. If your reception starts at a certain time, your Wedding Professionals will be setting up and preparing until that start time. Now, a professional usually will be set up and ready to go ahead of your contracted start time, but not that ahead of time - if your guests show up at 2:45 for a 4:00 cocktail hour, they could be in the way of deliveries or equipment being set up, or they'll be sitting around bored with the cocktail hour not ready to go. Plus if your Wedding Pros have to start working earlier than originally arranged for, it could alter their end time or you could get hit with overtime charges.
How to prevent it: Pay attention to the amount of time of the gap between the projected end of your ceremony and the start of the cocktail hour or reception, plus the estimated time to travel from one location to the other. If the time gap with travel is more than a half hour, or not enough time for people to go home and come back, look into local attractions such as museums, parks, tours, or other iconic sites to see in the area between the two venues where your guests could spend some time so they don't have to invade the reception venue too early. Include the suggestions (and directions) with your wedding programs to be handed out at the ceremony, or put it on your wedding website and include a line in the ceremony program to refer to the site for things to do until reception time.