The most common cause of maxillary central incisor to
be in cross-bite is
Which ever deciduous tooth may be retained beyond the usual eruption age of their permanent successor, is capable of causing:
i. Buccal/labial or palatal/lingual deflection in its path of eruption; 
ii. Impaction of the permanent tooth. Space occupied by the deciduous canine is less than the mesiodistal width of the permanent canines.
iii. The premolars might migrate mesially leaving limited space for the canines to erupt.
iv. It has the longest path of eruption.
v. Controversially, as it may seem, it is the only tooth to erupt after root completion.
TEXTBOOK OF ORTHODONTICS Gurkeerat singh Ed 2nd P:197
Premature exfoliation of deciduous maxillary central incisors
Prolonged retention of deciduous maxillary central incisors
High labial frenum
Early loss of deciduous mandibular