I couldn't help thinking there was something going on behind the scenes.

Literal

Behind [to-に] something exists [quotative-と] think without-doing [unable-にはいられなかった].

~ずにはいられなかった (couldn't help but) is a double-negative compulsion pattern: 'couldn't not think' = 'couldn't help thinking.' 裏に何かある (there's something behind it) expresses suspicion of hidden motives.