Intrahydrogen bonding
From the abstract to ref 1:


  We investigate both experimentally and theoretically the structure and photodynamics of nitrophenol molecules and clusters, addressing the question how the molecular photodynamics can be controlled by specific inter- and intramolecular interactions. Using quantum chemical calculations, we demonstrate the structural and energetic differences between clusters of 2-nitrophenol and 4-nitrophenol, using phenol as a reference system. The calculated structures are supported by mass spectrometry. The mass spectra of 2-nitrophenol clusters provide an evidence for a stacked structure compared to a strong O–H···O hydrogen bonding for 4-nitrophenol aggregates. 


The key words in the context of 2-nitrophenol are "stacked structure". This suggests that Mithoron's comment might be correct. However "stacking" is not evidence of a particular attractive interaction mechanism, it doesn't tell us of the relative importance of classical pi-stacking versus dispersion interactions between 2-nitrophenol molecules. 

I'll throw up my hands and claim ignorance, deferring explanation to the Wikipedia:


  In chemistry, pi stacking (also called π–π stacking) refers to attractive, noncovalent interactions between aromatic rings, since they contain pi bonds. These interactions are important in nucleobase stacking within DNA and RNA molecules, protein folding, template-directed synthesis, materials science, and molecular recognition, although new research suggests that pi stacking may not be operative in some of these applications. Despite intense experimental and theoretical interest, there is no unified description of the factors that contribute to pi stacking interactions. 


References


Photochemistry of Nitrophenol Molecules and Clusters: Intra- vs Intermolecular Hydrogen Bond Dynamics. Kateryna Grygoryeva, Jakub Kubečka, Andriy Pysanenko, Jozef Lengyel, Petr Slavíček, and Michal Fárník. The Journal of Physical Chemistry A 2016 120 (24), 4139-4146. DOI: 10.1021/acs.jpca.6b04459