The phrase "a second theory is" is correct and usable in written English.
You can use it when introducing an additional theory or explanation in a discussion or analysis.
Example: "While the first theory explains the phenomenon well, a second theory is needed to account for the observed anomalies."
Alternatives: "another theory is" or "a different theory is".
A second theory is that the pebbles are clods of dust and that the force of the air bag pressing down simply pulverized them.
A second theory is that workers are using disability as a safety net of last resort once their unemployment insurance runs out.
A second theory is that Mechel will be taken over by a state corporation run by one of Mr Putin's former secret-service colleagues.
A second theory is that Kennedy developed adrenal insufficiency as a youth, and that steroid treatment for colitis alleviated the undiagnosed adrenal condition, possibly leading to the spinal fractures and chronic adrenal insufficiency.
A second theory is that the escalation is coming from certain rooftoppers using the practice as a pathway into political activism – and the Hong Kong hijack and the Moscow star stunt certainly indicate this may be the case.
A second theory is that although people may not be working with a full deck of information and beliefs, their preferences are dictated by something, and that something is élite opinion.
A second theory was that the midget attempted to return to the mother submarines but exhausted its battery power before reaching the Port Hacking recovery point and would therefore be outside and to the south of Sydney Heads.
A second theory was that TRAPS mutations increase the affinity of the receptor for TNFα – a concept difficult to rationalise as mutations would be predicted to have a profound effect on the overall structure of TNFRI, and which is not supported by binding measurements of radiolabelled TNFα to patients' leukocytes [ 31].
A third theory is that the Russian government has some kind of hold over Trump.
A third theory is that the overture to Trump was designed to put pressure on Hillary Clinton to follow through on her earlier pledge to debate Sanders in California.
A third theory is that Darwin suffered from the psychosomatic stress of knowing that his theory of evolution would distress his dear wife and the God-fearing Victorian public.
