I have heard the interpretation that Æthelbald rebelled against his father due to his marriage to Judith (later Judith of Flanders) the daughter of Charles the Bald, who was at the time King of West Francia (only).
As her womb had been 'blessed' he may have feared that any sons born to her would have a better claim to the throne of Wessex than had Æthelbald.
However, the logical flaw I see in this interpretation is that Æthelbald would have known his father was elderly and had recently taken a 'pilgrimage' to Rome, both giving strong hints that his father would not live long.
Did Æthelbald really believe that a young child born of Æthelwulf and Judith, without any protection from his father, would be a threat to him? Was he afraid of Charles the Bald invading from across the channel to support his grandson? If so, was that a legitimate fear?
Full disclosure - I've been listening to the British History Podcast and this apparent logical flaw stuck with me.
Hi ActivisionBlizzard, thanks for posing this question and I'm really pleased that you're asking proper historians what their take on this matter is, rather than simply relying on the account provided by a history communicator (ie, me).
For the Historians looking into this issue, the initial source for this line of thinking comes from D.P. Kirby. There maybe be earlier historians who've taken this line (perhaps Stenton?) and off the top of my head Yorke might be also someone to look into (I'll confess I'm a couple pints in, so I can't provide exact citations, I'm just excited that people are discussing the show on here). But anyway, from my perspective as a history communicator, D.P. Kirby would be a good starting point for anyone looking into this.
[Edited to add] For people wondering what OP is talking about... here are the episodes that discuss Judith and her "blessed womb."
http://www.thebritishhistorypodcast.com/archives/2381 (direct audio link http://www.thebritishhistorypodcast.com/episodes/187.mp3)
http://www.thebritishhistorypodcast.com/archives/2387 (direct audio link http://www.thebritishhistorypodcast.com/episodes/188.mp3)