Recent experimental observations in β-Ti alloys have demonstrated the formation of a dense population of fine intragranular α precipates when the alloy is step-quenched below a critical temperature. These precipitates are associated with a sudden, significant increase in the apparent nucleation rate. We developed a computational model of α precipitation in Ti alloys, via the phase-field method, in order to investigate the fundamental cause of the observed microstructural changes. We simulated the nucleation of α phase in TiMo for a series of compositions and temperatures and qualitatively reproduced the experimental observations. These results are explained via a pseudospinodal nucleation mechanism: when the system is close enough to the critical point at which the α and β phases have the same free energy, fluctuation-assisted partitionless transformation from β to α becomes the dominant precipitation pathway. Consequently a rapid increase in the nucleation rate occurs, resulting in a fine distribution of numerous intragranular α precipitates.