The only things that can cause a transmission to seize is loss of lubrication or catastrophic mechanical failure.
Loss of lubrication could be caused by a drain plug to have fallen out or an oil seal that had leaked oil over a longer period of time. With lack of lubrication you usually get warning of the bearings starting to growl and difficulty shifting gears.
Catastrophic mechanical failure will be sudden, no warning. Something has come apart in the transmission internally causing everything to lock up. Did you experience any unusual noises or unusable driveability issues before the transmission locked up?