Translation:For the mouse, cheese is the only motivation.
It's because the sentence began with something other than the subject (in this case, a prepositional phrase) and as Dutch is a V2 language, the verb must be the second element in a main clause. So, the verb and the subject are inverted.
Note that one could say:
Kaas is de einige motivatie voor de muis.
In that case the prepositional phrase (voor de muis) has been moved to the end of the sentence.
Hope this helps.
Not necessarily true. I used to have a pet mouse that would eat doner over cheese any day of the week !