[kq76]

Interesting. Even taking the fact that the Mar/Apr and Sept/Oct include partial second months, it's still way more than you'd expect given the other averages.

As for "man-games lost" (that's the term I was trying to remember), I found a few studies.

One study says:

In conclusion, in the evaluation of MLB injuries resulting in time on the disabled list, there were a number of noteworthy findings. First, upper extremity injuries are predominant in pitchers while lower extremity injuries are more common in position players. ... Third, the highest injury rates were seen in the early months of the season and rates declined as the season progressed. ...

But, it does point out earlier that:

We elected not to include preseason exhibition games in our injury and exposure calculations, while clearly these injuries may result in regular season disabled list designation. In order to best control for this, we excluded players placed on the disabled list before and including the first day of the season; however, it is possible that some prevalent cases were included in our data that may have contributed in part to the higher observed incidence rate in April. Finally, players who are injured during the month of September may not be placed on the disabled list because of MLB rules permitting teams to expand their rosters to include 40 players. This may in part explain the much lower injury rate observed during the last month of the regular season in the present study.

IOW, the early and late season data may not be telling the whole truth.

There's also this study that specifically looks at hamstring injuries and it seems to agree that they're more often seen in the early part of the season. As the article says, "Hamstring strains are the most common injury for professional baseball players and can result in significant time on the disabled list.". Injury rate is the # of injuries per game so the lower the #, as seen in April and May, the more common.

And this study, about shoulder injuries, concurred. It also claimed the shoulder was, "the most vulnerable anatomic location".

The 511 injuries were spread along the months of March September, whereas October–February witnessed no injuries (Figure1). April was shown to be the month of the most injuries with 125 (24%) injuries, while June and May came in second and third with 89 (17%) and 81 (16%) injuries, respectively. Injuries that required surgery were highest during the months of March and April with 24 (50%) and 17 injuries (35%) respectively (Figure2). Moreover, we calculated the average number of days the athlete was placed on DL due to shoulder injury by month (Figure3). A decreasing trend is evident showing March and April to be the months with the highest number of days issued on the DL (97 days and 92 days respectively), whereas August and September showed the lowest number of days issued on the DL (26 days and 31 days respectively).

I'm sure there are more studies on the topic, but at this point I'm willing to just go with there are more injuries in April, whatever the reason, and so maybe it should have an extra off day or two.