This is speculation but I think it's accurate. Spells carry a certain energy which will do something specific on impact with a target. For example, Avada Kedavra has the potential to silently and immediately kill a creature it hits. This is curious, unusual and very powerful. Now, if there is nothing to kill and it collides with something else, it stands to reason that that energy or magical potential (or whatever you might want to call it) needs to be expended. It also stands to reason that this spell should be quite destructive.
I think the reason that they are destructive/explosive against inanimate objects but less obviously violent when they hit a designated human target is that when hitting a human a specific effect is unleashed - which may just be being magically forced to laugh or fall over. But, if this magical effect literally cannot happen because it is trying to, for example with Riktusempra, cause laughter but there is no mouth, no brain, no vocal cords - then that energy is expended as bangs, light and explosions.
So, send a specific spell at a human or animal and get the specific outcome, but send it at a wall or table and it cannot do its job and a violent outburst of magic is the effect.