Would Freddie Gray have intentionally thrown himself around the van to get beaten up and thus get sympathy/medical treatment instead of a jail cell? Probably not because that would be sort of stupid, but I don't know him or his background to say for sure. Could he have? Sure possibly, and without other evidence to settle the question that means a reasonable doubt exists as to whether the driver was responsible.
Please recall that 'reasonable' in law doesn't mean 'what the average person considers sensible' or suchlike, but a doubt that can be raised by using reason/logic. So you could argue that the victim's injuries were inflicted by passing ninjas, for example, but you'd have a hard time coming up with a logical explanation of how that might have happened.
On the other hand people have been known to injure or kill themselves deliberately even though the reasons for doing so don't necessarily make sense to others; this is a fact. We can't ask Freddie Gray because he's dead, sadly, and as a result a doubt exists about how he incurred his injuries absent other evidence. Where a doubt exists you can't have a conviction.
This is one of the difficulties of the US being a common law country; our trails are about establishing whether a sufficient burden of proof has been met to impose criminal liability. In some other countries that use a civil law system trials are conducted more like inquests, with the highest priority being to find the truth of what happened and allocation of responsibility and punishment a secondary objective. One interesting side effect is that a system like this eliminates plea bargaining. If someone is murdered in Germany, for example, the trial is going to take place regardless of whether the prime suspect admits guilt or not; even if the person freely accepts responsibility for a crime the trial is carried out anyway to establish a record of what actually happened. We could do worse than incorporate that idea here.
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