When you see racking like that up close...heavy duty it doesn't look
.
The FLT possibly weighs 2 tonnes, it possibly caught the upright with his load guard and considering the beams are only a couple of mm's thick and the uprights are only 2 or 3 mm thic...maybe 4, they don't take much to damage them. Plus it relies on all the force being downwards through the uprights, disturb that and you have a big problem.
There's a lot of what looks like fluid spraying out as it goes, and fluid is heavy so they may have overloaded it....but it may well have been specced for the weight that's on it...depending on where in the world this happened.
Drive in racking is stronger, it's a mass of interlocked bays, very strong but harder for FLT drivers, you get more damage but it can take a lot more.