Very common. Tri-shoes are designed for just such a use and appropriately so. The strap on a tri shoe hangs away from the bike when undone while a typical road shoe is on the inside. Because triathletes are "transitioning" from the swim and to the run it allows us to get our feet in and out easier and safer - away from the wheels and gears. Road shoes on the other hand are strapped once before the race and never undone until after.

You might also notice that some triathletes shoes tend to stay horizontal while the racer is running next to the bike. This is with the help of rubber bands that hold the shoe in place for easy foot placement once the racer mounts the bike. The rubber band just snaps and falls away once the foot starts to pedal.

Little things like that help us to save time in our transitioning. Not everyone does these but they are very common.