There is always a reason that a toddler will start screaming even though it may seem like there is no reason for the toddler to be screaming.
Toddlers are really full of energy and some toddlers will scream and throw a tantrum just to get the parents or a siblings attention and to agitate them.
If your toddler is screaming for no apparent reason then don't give into them because it will teach them that if they scream and throw a tantrum that they will get what they want and the screaming and tantrums will continue.
Just like if a toddler has tantrums which is normal just be calm and don't give the toddler too much attention but make sure they don't hurt themselves.
The toddlers will naturally have their voice up high even though they don't mean too and they are not always doing so to annoy you.