Why are ULXs so often associated with colliding systems? Is it simply because galaxy-galaxy or cloud-galaxy collisions produce a higher star formation rate, and therefore more young X-ray binaries? Or is there something fundamentally different in the way stars and compact remnants are formed in those environments? The Sc galaxy M99 in the Virgo Cluster may be a good test case; we discuss this using XMM-Newton, HST, and the VLA observations. M99 has been strongly affected by tidal interactions and recent close encounters, responsible for an asymmetric spiral pattern and a high star-formation rate. Our XMM-Newton study shows that the inner disk is dominated by hot plasma at kT ̃ 0.30 keV, with a total X-ray luminosity ̃1041 erg/s in the 0.3-12 keV band. At the outskirts of the galaxy, away from the main star-forming regions, there is a ULX with an X-ray luminosity ̃ 2 X 1040 erg/s and a hard spectrum well-fitted by a power-law of photon index 1.7. This source is close to the location where a massive HI cloud appears to be falling onto the M99 disk at a relative speed > 100 km/s. We speculate that fast cloud collisions may trigger large-scale dynamical collapses of molecular cloud cores, leading to the formation of very massive (>200M[☉]) stellar progenitors; we argue that such stars may later collapse into massive black holes (̃100 M[☉]) if their metal abundance is sufficiently low. Thus, we suggest that there is a direct link between collisional environments and ULX formation.