Welcome to FT!, Estefania Cunha Haas
While an aircraft change could be one reason there are many other reasons.
It would help to have a date to look more specifically, as the dates I checked look fine.
Also, are these Basic Ecomony or regular Economy tickets? Do they have access to Economy Plus?
Other reasons
For those without EconomyPlus access, the seats may be soldout.
Additional UA blocks seats for those with medical issues.
UA also blocks seats to be able to accommodate families so that can sit together.
All these will be freed up at checkin