The new mining barges offer far less of a yield difference between T1 and T2 variants than they used to (compare current vs former Retriever and Hulk), all T1 barges got their prerequisite training routine drastically cut (all are now barge 1 plus astrogeo 3, then compare that to the former barge 5 and astrogeo 5 for the former Covetor), and mining in a Retriever or Mackinaw is pretty much almost as good as jetcan mining before in a Hulk, but without the risk of can-flipping... which basically means mining should become HUGELY more popular than it was before (at least in the short run, in the long run, not really that much), and with a drastically reduced barrier to entry.

People that are likely to engage in mining are UNLIKELY to be PLEX sellers to begin with, they are far more likely to be PLEX buyers. Also, a good portion of former serious miners would have already had a Hulk, which they can still sell off at a decent price and only add a bit more to afford a Mackinaw (they can even buy a Retriever as an intermediary, the current Hulk market value more than covers quite a few of those). Or, you know, just get a Retriever either way, even if you had no Hulk to sell, a Mackinaw is not THAT much better if you can be bothered to be even remotely semi-active.

At the same time, the mere ANNOUNCEMENT of technetium alchemy has dropped technetium prices (they're in a bit of a short-term rebound, but it will keep on dropping), which means in the long run, exhumers will end up being noticeably cheaper - which is an added incentive to just go with a Retriever instead of a Mackinaw while price have a time to settle down.
On the other hand, mineral prices are also likely to go down slowly, so in time, some that did re-pick-up (or just newly picked up) mining will probably get bored of it. And less tech moongold revenue probably will also mean less alliance-subsidized PLEX or alliance-subsidized ships, which might increase supply of fresh PLEXes (from people that start buying more of their stuff instead of getting it from the alliance) and decrease demand for PLEX (as some players might start dropping less useful alts that used to be alliance-subsidized to some degree). Probably not a very huge change, but still a change.
...

All in all, there's no much reason to suspect a huge SHORT-TERM influx of PLEX sellers due to just those factors above, but a good reason to expect a significant SHORT-TERM demand increase in form of PLEX buyers due to just those factors above, which basically means price sort of has to go up to compensate and rebalance supply with demand.
Long story short, the MOST LIKELY MAIN CULPRIT is the mining ship redesign, with technetium alchemy possibly relevant but not greatly so, and possibly several other smaller-effect factors also playing some even smaller role in it all.
It probably won't last very long, but it's not sure how much time until a re-normalization will happen, or if it will happen at all soon.