Some children do become more aggressive and extremely stimmy when beginning any type of behavior therapy (not just ABA). Any change in your child's day is going to cause this.
If the behavior continues being a real problem for a long time, you might want to consider a different approach. My son was bored to death with ABA, but when we switched to the floor-time method (Greenspan), his response and progress was amazing to watch.
I think that the type of therapy your child receives should depend on what works for your child, not what anyone says is "right." Obviously, if your child freaks out and is extremely distressed every time they see their therapist, it's time to reassess which approach is the right one.
Just my two cents! I hope the therapy works out great, though -- just give your child some time to adjust. It does take some time for them to get used to these demands being placed on them.