i'm no physicist, just a layman offering a layman's opinion, but i don't think it was vandalism either. if it was a bat there'd be much more damage and splintering.
in tanks with curved corners (i.e. a tank with 3 viewable sides made from one piece of glass) it's possible the perpendicular "panes" were not perfect 90 degree angles. this can happen also with tanks made from 5 separate pieces (4 sides and a bottom), but the silicone jointing can alleviate some of the stress that results from slightly off angles because the silicone has more flexibility.
Even though it sounds counter intuitive, glass is flexible to a certain degree. and it's also technically a super super slow moving liquid (which is why, if you look through windows of really old houses, like 70+ years, and the view out the windows can look wave-y or distorted, it's because the glass has sagged a little in that time...maybe modern glass is different, i don't know).
so, if the perpendicular sides are off a degree or two, and then are force fastened to a bottom pane and/or back wall which causes some flexing, it can put stress on the glass. so, a small impact, or even just time, can cause the glass to crack at the stress point.
and, even though the glass looks "sucked" in at that break point, it may not be from the force of an impact, but rather the glass returning to its unstressed or resting position. that and add in the weight of all the water above the crack pushing outwards and the force of the water escaping, could also explain why the top part of the crack extends out.
i'd call the tank manufacturer and try to get your money back or a replacement.
that's my .02.
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looks like Dapg8gt already said what i said, just way more efficiently.