As an atomic object, MatchQ[1/2, HoldPattern[Rational[__]] and MatchQ[1/2,Rational[a_,b_]] never really should have worked. Compare, for exampe, with these examples:
In[23]:= MatchQ["asdf", String[_]]
Out[23]= False
In[24]:= MatchQ[3., Real[_]]
Out[24]= False
The fact that they did was a quirk of implementation, which changed to a more efficient data structure in V10. Because of how common Rational[_,_] is, we continued to support by a special hook, but arbitrary patterns inside of Rational are unlikely to work. (BTW, the same thing happened with Complex in V9).
Fortunately, it should only be Rational is affected. MatchQ[tMax/2,HoldPattern[Times][Rational[n1_,n2_], b_]] and MatchQ[tMax/2,HoldPattern[Times][a_Rational, b_]] are both good replacedments that will continue to work. Notice also these don't require an inner HoldPattern as the Rational involved won't even try to evaluate.
